If you are not sure what Twitter is then go and check it out. This page contains some notes on helping you get the most out of it.
First off, always look out for the "Verified account" icon (blue with a white tick), these are generally significant, "public interest" accounts that have been verified. You can see someone's first Tweet at Discover your first Tweet | Discover, this allows you to compare when the first tweet was sent with the account creation date and can also help show if an account has been changed significantly. When trying to identify spam, junk or parody accounts watch out of 1's instead of l's they can sometimes be hard to distinguish. You can also use Twitter Advanced Search and you can set a date range, for example, to check an account out.
I also find Twitter Analytics useful for seeing how popular my Tweets are.