It's not too hard to figure out. For the most part it's just one big chatroom.
Let me see if I understand this: "@" means you tagged that account in your post and "#" is when you write in a topic?
When you use the @ sign before a username, it lets that user see what you have written by clicking the Mentions button (or the @yourusername). Putting a # before something has marked it as a topic, so when you click it you can see every post on twitter that relates to that #topic.