Aside: I’ll be honest with you; this article has been written by men much smarter and more studied than I could hope to be on either side of glory. But Drew doesn’t pay me to link to other people’s stuff,  so here’s my stab at it.
A lot of people throw around the term “Reformed” nowadays. But some people who use the term think it’s just a synonym for Calvinism. Calvinism is certainly part of Reformed Theology; it’s impossible to be Reformed without it. But there is a lot more to it than that.
From here I’ll explain some of the distinctives of Reformed theology. This is not meant to be comprehensive, nor is it meant to prove Reformed theology Biblically. This is merely meant to explain it.
Reformed Christians are Confessional
To be confessional means that you subscribe to a confession of faith that you believe accurately summarizes Christian teaching. In this case, to be Reformed means you agree with one of the sets of Reformed confessions.
The three primary sets of Reformed confessions are as follows:
The Westminster Standards (Presbyterian)
The Three Forms of Unity  (Reformed )
The Baptist Confessions 
- 1689 Baptist Confession of Faith (or the Second London Baptist Confession)
- The Baptist Catechism (or Keach’s Catechism)
- An Orthodox Catechism
Out of the above, I’d recommend reading the Heidelberg Catechism or the Orthodox Catechism (which is a Baptist revision of the Heidelberg). Those are the easiest to read. The others tend to have extremely long sentences, really old language, and complex wording, but they’re worth it if you want to dive in.
Note what I said before I listed the confessions: Reformed Christians subscribe to these confessions because they believe they accurately summarize Biblical teaching. This is not the same as Roman Catholicism, where the church is viewed as having the same authority (or more) as the Bible, or where the Catholic catechism is viewed as authoritative teaching. The Reformed confessions are not equivalent to the Bible nor a substitute for it. The confessions themselves confess the Bible as the highest authority. Rather, they are meant to help us understand it. The church does not determine the Bible’s teaching; it discovers it, summarizes it, teaches it, and most importantly submits to it.
One also may not know all of these confessions by heart, or even know that they exist. In my mind, if one finds substantial agreement with them, he can call himself Reformed even if he thinks that the Canons of Dort are gigantic weapons.
People debate over whether one has to subscribe to every word of a confession to be Reformed. I won’t address that debate here, but Reformed and Presbyterian ministers are typically allowed to take minor exceptions to the confessions when they’re ordained.
Reformed Christians are Calvinists
Like I said above, Calvinism is not Reformed theology. But Reformed theology contains Calvinism.
What is Calvinism? Calvinism is typically summarized by the flowery-yet-unfortunate acronym TULIP. It’s unfortunate not because the doctrines aren’t true, but because the terms are easy to misunderstand.
Total Depravity (or Radical Corruption ) doesn’t mean that we are as evil as we could possibly be. It means every part of our being — our thoughts, our actions, and our words — are corrupted by sin. We are also so depraved that we hate God by nature and want nothing to do with him.
Unconditional Election (or Sovereign Election) means that God has elected to save people without them deserving it in any way. He saves people simply because he wants to. Those people are called the elect.
Limited Atonement (or Definite Atonement) is the one most people get hung up on. It means that the death of Christ fully paid for the sins of all the people throughout history who believe in him. Conversely, his atoning death was not meant to pay for the sins of anyone who doesn’t and never will believe.
Irresistible Grace (or Effectual Grace) means God changes the hearts of the elect such that they will repent of their sins and believe in him. The change isn’t one of dragging people kicking and screaming into the kingdom; it’s one of changing them from people who hate God to people who love God and want to be in his kingdom.
Perseverance of the Saints (or Preservation of the Saints) means that God will keep his elect throughout their lives. They cannot lose their salvation. This doesn’t mean they can run off and live like hellions and expect that they’ll be okay when they’re face-to-face with God; rather, God’s elect will continue to be sanctified throughout their lives.
As for why we stick to the easily-misunderstood terms rather than the better ones…well, would you rather have a TULIP or a RSDEP? That’s what I thought.
Reformed Christians are Covenant Theologians
Covenant theology is a complicated subject, one about which entire books can (and have) been written. So I’ll give my pathetic attempt to summarize it here. Covenant theologians believe in three covenants which drive the Biblical story.
The Covenant of Redemption is the covenant that the Father and the Son made in eternity past. The Father promised to give the Son a bride, the church, on the condition that the Son die for them to pay for their sins. The Son agreed to this.
The Covenant of Works is the covenant that God made with Adam in the garden of Eden. The covenant stated that, if Adam passed God’s test to obey him (in Adam’s specific test, to not eat of the tree of the knowledge of good and evil), he would attain eternal life for himself and all his posterity. Adam, obviously, broke this covenant, and represented all of humanity when he did. Thus, we are counted as having broken the covenant.
The Covenant of Grace is the covenant God made with his elect to provide Jesus to fulfill the Covenant of Works on their behalf. Jesus had to live a perfect life on earth in their place, and he did. Thus, God counts us as having fulfilled the covenant.
People debate about how the covenants we see in the Bible — the Abrahamic covenant, the Noahic covenant, the Mosaic covenant, the Davidic covenant, and the New covenant — all relate to this structure. That’s a whole different post.
Covenant theology also contrasts with dispensationalism. We believe that Israel and the Church are not two separate entities, but the same. The Church is true Israel, composed of all Old Testament believers (such as Moses and David) and New Testament believers and beyond.
This is often errantly called “replacement theology.” But the church doesn’t replace Israel; the church is Israel. The church wasn’t born in Acts 1; it was born in Genesis 1.
Yes, this also means you can’t be a dispensationalist and Reformed. Sorry, John MacArthur.
Reformed Christians Believe in the Five Solae
The five Solae are actually a Protestant distinction rather than a Reformed one. But given that this is the 500th anniversary of the Reformation, I figured we should talk about them.
Basically, we believe that sola Scriptura is our highest authority. We believe we are saved by sola gratia through sola fide in solus Christus to soli Deo gloria.
Confused by my butchering of Latin? It means the following:
Sola Scriptura or Scripture Alone. Scripture is our only and highest authority.
Sola Gratia or Grace Alone. We are saved by the grace of God alone.
Sola Fide or Faith Alone. Faith, not works, is the only instrument through which we are saved. When God enables us to place faith in him, his grace saves us.
Solus Christus or Christ Alone. Only Christ saves. Not the Pope, not the President, and certainly not the PlayStation (without a memory card anyway…and I just aged myself).
Soli Deo Gloria or To God Alone be the Glory. That’s self-explanatory.
Just a side note: sola Scriptura does not conflict with confessionalism. Remember, the Reformed confessions derive their content from the teaching of Scripture. In fact, Martin Luther, the man who set off the Reformation, wrote a confession and two catechisms. Clearly he didn’t think those conflicted with him being held captive by the Word of God.
Reformed Christians Read the Bible Christocentrically
That’s a long word for “with Jesus at the center.” When Reformed Christians read the Bible, we read it with Luke 24:44–47 in mind:
Then he said to them, “These are my words that I spoke to you while I was still with you, that everything written about me in the Law of Moses and the Prophets and the Psalms must be fulfilled.” Then he opened their minds to understand the Scriptures, and said to them, “Thus it is written, that the Christ should suffer and on the third day rise from the dead, and that repentance for the forgiveness of sins should be proclaimed in his name to all nations, beginning from Jerusalem.” (ESV)
In other words, we read the entire Bible knowing that all of it is about Jesus. That means that, in the story of David and Goliath, you are not David, and Goliath is not your problems. You are the Israelite cowering on the sidelines while Jesus battles the enemy on your behalf. Again, entire books have been written and courses have been taught on this subject, but Jesus is on every page of the Bible. Plain and simple.
To fully express the distinctives of a Reformed theology would take an entire series of posts, most of which I’m not qualified to write. But I hope this helps explain what it means to be Reformed.
 Actually, Drew doesn’t pay me at all. I do this out of the goodness of my heart.
 Church historian Dr. R. Scott Clark has referred to the Three Forms of Unity and the Westminster Standards collectively as the “Six Forms of Unity.” This is because these confessions and catechisms, while different, harmonize together.
 This is one of the confusing things. There are two kinds of Reformed: one is a brand of theology, and one is a group of denominations. They come out of the Dutch Reformed tradition specifically. In this article, I’ll refer to the brand of theology.
 There’s a raging debate about whether Baptists can be Reformed or not due to having a different understanding of the sacraments (i.e. they don’t baptize babies) and a different understanding of covenant than more traditional Reformed people. I won’t address that debate here.
 I give credit to R. C. Sproul for these alternate terms.
 Yes, this assumes that the fall had been ordained before Creation actually occurred.
 You may question my qualification to write this post, too. That’s okay, we can still be friends.