You may get excited when you have just joined twitter and many people started following you or when you suddenly get many people following you within a short period of time, unless you are not a celebrity
If you want to know what type of people are following you, or if at all they are spammers or robots, who just came to spam twitter for any reasons. Twitblock is a nice tools which scans your twitter account to find these kind of people. You can only scan your account with this tool as it requires twitter account (OAuth) authorization before scanning. After scan is complete you can just block these accounts. If you know some people that are not spam and still coming is this list, you can click not spam. A screenshot of report of my account is shown below.
I have found out that my account has 94 such followers. All these people are not spam definitely. Some are new accounts that have just joined twitter and some with not many tweets from their accounts.
You can also help stop spam on twitter by following @spam, and sending a direct message containing the username of the spammer to this account.