When you save a link with Bitly it comes with a 6-7 random character ending. You can customize the ending of this link to something easier to remember or type. This is sometimes referred to as a vanity URL.
For example, you can take a link like this: http://bit.ly/YpsrII
And customize it to look like this: http://bit.ly/bitly_signup
You can customize a link right after you save it. You'll see a small pencil next to your Bitlink. Click on the pencil to edit the customization for your link. When you are finished, click 'enter' on your keyboard.
To customize a link that you've previously saved go to 'Bitmarks' located under 'Your Stuff' view (the landing page once you log in to bitly.com)You'll see a small pencil next to your Bitlink. Click on the pencil to customize your link. When you are finished, click 'enter' on your keyboard.
Remember: you have to create a unique customization that nobody else has used before. Customizations are case sensitive.