It seems that many Twitter users are desperate for additional followers, and they are willing to take offered shortcuts to achieve that goal.
Enter the bogus gain-more-followers services. Hosted on domains like obtainmorefollowers.com, moretwitterfollowers.com, purchasetwitterfriends.com, and similar, they offer to add a certain amount of followers to your account, sometimes even for a fee:
In order for “them” to effect the changes, you are asked for your Twitter username and password. If you don’t know it by now, giving up your credentials to any online account to third-party websites is not a good idea.
In this case, your account can be hijacked and used for spamvertising – that is, you will be promoting the service on your account and be responsible for duping some of your followers. You’re lucky if you haven’t lost any money in the process and the password isn’t changed, so you can change it yourself and shut out the spammers/scammers.
As Graham Cluley says, you’re better off examining the things you say and finding a way to make your tweets more interesting, that using suspicious services that could – and will – compromise your account.