So there has been an ongoing issue with people's Twitter accounts posting various things without their knowledge - usually advertising links ("I lost 20 lbs in two weeks, click here to find out how!")
As usual, NEVER EVER CLICK THE LINKS! I cannot stress that enough - chances are it will take you to a site that will try and install malware on your computer without your knowledge, or will ask you for some sort of login credentials.
So what do you do if this happens to your account? Follow these simple steps:
1) Login to your account on Twitter.com
2) Click "Settings"
3) Click "Connections"
4) Revoke access to everything listed here unless you are 100% sure you want it to have access to your account.
5) Click "Password"
6) Change your password - choose a complex password!
7) As a courtesy to your followers, send a quick tweet letting them know NOT to click on the links you had sent out.
Always be aware of what sites and applications are asking for you to log in with your Twitter username and password - be very clear on why that site wants it and what it will do. The only authorized connection I have is TwitterFeed - this service will automatically post a pre-formatted tweet when I have updated a blog. When the Shorty Awards were happening, I granted them access for a day or so, and then revoked it.
In the past, a number of people posted their "Twifficiency score" - this is a system that is not capturing your userid and password but you do grant it access to tweet on your behalf. Outside of the initial tweet of your score, what else will it be tweeting?
Sometimes you may not realize that you have granted access to your twitter feed - go in every once in a while and see what is listed under Connections.