1 and 2 Corinthians are epistles, which is just a fancy name for a letters. That means that what you're reading are two letters that Paul wrote a group of Christians in Corinth, right?
Well, not exactly.
Most scholars think that these two books don't just make up two letters, but that there are actually multiple pieces of correspondence that have been shoved together here. Our best guess is that 1 Corinthians is one complete letter from Paul, while 2 Corinthians is actually made up of two separate letters. Chapters 1-9 are the first letter and 10-13 are the second. Some scholars argue that 2 Corinthians contains as many as five or six different letters. Now that's some ambitious editing.
Confused yet? We thought so. So, let's add some more info onto the pile. These two texts don't even represent all the letters Paul wrote to Corinth. Some of his stuff has been lost forever. Along with the Corinthians letters back to Paul. It's sad. But, parchment just doesn't age well.
So how many letters did Paul write to the Corinthians? Well, no one knows for sure, but we can sure take a guess. (Oh, we love guessing games!) Here's a basic breakdown:
Letter A—Paul writes to the Corinthians sometime between 51 CE and 54 CE This is after his first big stay in Corinth when he establishes the church there. Paul tells the group "not to associate with sexually immoral persons" (1 Corinthians 5:9). Naturally, they misunderstand the heck out of this.
Letter B—This one is probably 1 Corinthians, which was written between 54 CE and 55 CE. The folks in Corinth are confused and Timothy does not have good news to bring back. Things are not looking good. Paul is in damage control mode.
Letter C—Because of all the troubles, Paul visits Corinth himself. Things do not go well. Maybe that's why Paul calls it "the painful visit." He then writes this "tearful letter" to the church in Corinth. Cheer up, Paul. It gets better.
Letter D—Problems solved! The Corinthians take Paul's letter to heart and seem to change their ways. In 55 or 56 CE, Paul writes in another letter what became 2 Corinthians 1-9 to congratulate the church on its newfound repentance. High fives all around!
Letter E—Not so fast. A few months later things have gotten bad. Real bad. Paul writes what is now 2 Corinthians 10-13 and is not happy one bit (source, pp. 1093-94).
So even though these two books make up a lot of awesome tidbits of Paul's wisdom, there are even more goodies out there that we'll never be able to read. Did Paul reverse his position on slavery? On women talking in church? On speaking in tongues? The world may never know.