The MIT License is a permissive license that is short and to the point. It lets people do anything they want with your code as long as they provide attribution back to you and don't hold you liable.
jQuery and Rails use the MIT License.
The Apache License is a permissive license similar to the MIT License, but also provides an express grant of patent rights from contributors to users.
Apache, SVN, and NuGet use the Apache License.
The GPL (V2 or V3) is a copyleft license that requires anyone who distributes your code or a derivative work to make the source available under the same terms. V3 is similar to V2, but further restricts use in hardware that forbids software alterations.
Linux, Git, and WordPress use the GPL.