To discuss is to talk: but with a more serious intent. Discussing is a lot more substantial than chatting.
When someone says, "I have something I need to discuss with you," most people think "Uh oh." To discuss usually implies a certain amount of serious talking. This can apply to writing as well: essays discuss topics, usually in a careful, thorough way. On the other hand, sometimes discuss means exactly the same thing as talk or converse. Like all words, the meaning depends on how it’s used.