You will get an email when somebody follows you but Twitter has no default service for tracking unfollowers but there are various tools available that helps you in finding out the person who unfollowed you.
Here I am listing few tools which help in catching twitter quitters for you:
Want to know who bid goodbye to you on twitter? Then follow @goodbyebuddy. It helps you in checking un-followers stats. All you have to do is follow @goodbyebuddy on twitter and visit its page goodbyebuddy.com. It lets you know who un-follows you and when you are being un-followed.
When someone un-follows you on twitter, Qwitter sends you weekly email notifications about who unfollowed you and at what tweet (might be the reason behind unfollow) did he/she un-follows you though I have not received any alerts from them for quite a long time. You just need to enter username and email.
It is another service which helps you in catching twitter quitters by sending you an email every day. All you need to do is join it by entering username and password.
@unfollowr is a twitter service that helps you to track people that stopped following you. You will receive digests to your email or direct messages with list of unfollowers (including accentuated ex-friends) or link to a digest as direct message in twitter. All your need is to follow @unfollowr.
Friend or Follow is a service which shows who you are following and who are your fans and friends. You just need to enter your twitter username and you will see thumbnails of all twitterers who have not reciprocated by not following you back and vice versa. You can sort the list by username, account age, location, followers, following and last tweet.
Who unfollowed me is a twitter service that helps you track unfollowers, who is not following you back and who you are not following back. All you need to do is sign in with your twitter account.
Byebuddy is a service which tracks unfollowers and current followers. All you need is to sign in with your twitter username.