Twitter is a great place and its really cool to have many twitter followers. But a common problem with twitter is that lots of spammers / bots follow you, and after sometime they will un-follow you. There is no feature in twitter to know who has stopped following you.
But with use of a third party free service its easy to know and get notified by mail when someone un-follow you on twitter. Its Qwitter, a free service that sends you daily email notification showing all your Twitter unfollowers. Its a great free service that helps you to identify Twitter spammers and spam bots.
Activating Qwitter service is very easy and will hardly take 2 minutes. Just enter your Twitter username and password, they will send you a mail with a link to activate account. Click the link sent in mail, and your account is active.