Just like 1 Thessalonians, this letter was written to the church in Thessalonica, and believers still seem to be struggling with very similar issues - some of them thought that Jesus had already returned and they'd missed it. This letter is written in a more formal style than the first one. Some theologians think this might suggest that it was written by someone other than Paul, although it's possible that Paul is just trying to say things more clearly. As in the previous letter, Paul is trying to make clear some of the basics of the Christian faith to a bunch of relatively new and uncertain Christians.
Some of the Thessalonians were so focused on the end of the world that they had forgotten to live now, so in chapter 3 Paul has to tell them not to be lazy. He reminds us to balance the 'now' with what God promises for the future. Like 1 Thessalonians, it's also a great letter for new Christians (or people exploring the faith) to read to learn about some of the fundamentals of Christianity.