I wanted to add to this as there is no link to where to get the GApps. The newest group is Open Gapps. These do not work with EVERY custom ROM, but the vast majority (and those based on AOSP should work).

Open Gapps: http://opengapps.org/

This site is awesome because when loaded you get a basic UI with a couple selections that allows you to specify what you want (some packages have more google apps, others have less), they even have an option to include the Aroma Installer version which when flashed in TWRP or clockwork will actually bring up a menu system which you can tap to make selections of which apps and which settings you would like at the time of the install! Of course it is large as it includes everything, but it can be nice to be able to customize the gapps install as you are installing it (and yes it has the ability to save your selections for future installs at the end of the setup UI it will ask if you want to save your selections. If you do future installs it just opens the UI and you tap to load the saved file and done).

Open Gapps has help ?s all over and documentation on their site to help you with many questions.

Cyanogen MOD has their own version of Gapps (page lists gapps packs for every version of CM ROM from CM4+ on this page. Note that as of CM11 CM Roms can now flash Open Gapps.):

https://wiki.cyanogenmod.org/w/Google_Apps
(looks like as of CM13 they have stopped their custom Gapps zips and now fully use Open Gapps).

With these 2 links you should be good to flash GApps with just about any version of Android (I can not account for a severely custom job done by someone that may not work with Open and/or CM GApps, as we know what happens when we assume in the tech world!).