Gmail has for quite some time offered an option in its Labs section called "Undo Send", but as most people don't traverse there, not many ever used it. But it goes without saying that many people would take advantage of it at some point, and it's for that reason that Google has added the ability to enable the feature from the standard settings menu.
With the featured enabled, you'll see a prompt at the top of Gmail after sending an email that will allow you to fetch it back. This of course doesn't delete the message from the target, but instead prevents it from actually being sent until a time-limit is reached. By default, this threshold is 10 seconds, but you can set it to 5, 20, or 30 seconds.
To access the setting, you can click on the gear to the top-right of your Gmail. You'll find it beneath the "Send and Archive" option and above "Stars".