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.
My first ever Tweet was Geoff Lawrence on Twitter: "Just created my account, time to sort out pic, bio etc!" / Twitter just in case anyone is interested, it was posted at 14:48 on Oct 30, 2010 but I created the account earlier at 13:46:56 on Saturday 30th October 2010.
I also find Twitter Analytics useful for seeing how popular my Tweets are.
The starting point for development with the Twitter API is Twitter Developer Platform — Twitter Developers and Twitter recommend using twitter/twurl: OAuth-enabled curl for the Twitter API.
It is well worth going through these, every now and then just to see what has changed. I recently found Interests from Twitter / Twitter which was worth adjusting!
Some automated accounts, or bots on Twitter spread harmful messages, however Bot Sentinel - Dashboard does a reasonable job of trying to detect these, it is well worth looking at.