If you didn't know, Twitter is a social network set up based a bit like updating your status on Facebook. You have 140 characters to write down a status(tweet). Your 'Tweet' can be an update on what your doing, a thought, an oponion or just something random!
Now twitter has got so large it is used by a a various amount of celebs in the wide world. Some just use it to self promote whilst others use it go get their opinion out there quickly. A lot just use it to let you get an insight into their world that you wouldn't normally get. Also you get the chance to communicate with them since every tweet where you mention their name in, reaches them.
Some sports stars are now banned from twitter for comments they have put on twitter and a lot of what they tweet is now carefully thought about beforehand.
These are only a taster, there are loads more out there!!
Sports stars who have twitter are: