Several sites store a page's state inside the URL.  They do this so that the page state can be passed along just by using the URL.  The problem is that when you try to email these long URLs, email clients will often times insert line breaks in the middle of the URL. 

The end result is that the person you sent the URL to in the first place, will get back to you and tell you that you sent an invalid URL (or they will simply just ignore the email).

To get around this problem, you can use URLZip to "zip" your URLs into small URLs.