Don't worry—epistle or is just another word for a letter. But Romans is different from most letters. It's actually even different from Paul's other letters, in that the bulk of the message doesn't address any specific questions or concerns from within the Roman community. In other words, it's not clear that the Romans have actually asked Paul to write to them at all.
Some scholars have referred to Romans as a "letter-essay" because it seems to be more of an extended dialogue or diatribe on issues that are running through Paul's head at the moment. The point of all this is clearly to convince the Romans that Paul knows what he's talking about and to win some support for himself once he finally arrives in Rome (source, 147). Good luck with that, Paul.