one project for many clients

What do you call an iOS project that I can export into a .a or library and reuse it with different projects.

For example, I have the base project that will have geolocation and twitter feed from a twitter account. I will compile this into a bundle with all of its core codes. Then on Project-A, I can customise with different images for background, button images, etc. Then on Project-B different resources also. Then I just import the .a into Project-A or B to make the geolocation work and twitter feed with custom values like twitter.username = @"russel-peters". And if I need to change some functionality in the library, I edit it export it into a bundle and reimport it to project A or B.

I hope you get what I mean, I do not know what to call this strategy in iOS development so I could not make a research for it.