Twitter lingo

Ok, so this post may make me sound a bit ignorant, but that's alright. I figure that if I don't ask I'll never learn. I'm looking for some help with twitter lingo. I read tweets and wonder, "What does that mean/" all the time. For instance, what does it mean when there is a # in front of a site address? What does RT mean? What does the @ symbol in front of an address mean? What does the number above my message box stand for and why can I sometimes leave long messages with no problem and can't tweet short messages? And how do you link an address into your tweet? If anyone can help me I would greatly appreciate it. Thanks in advance.