Understanding custom android launcher

Your best bet is to start by looking at the sample launcher provided by Google. You can find that in SDK folder:

[Android-SDK]/samples/android-x/Home/

(where x is API level).

They have provided source code for an example home screen and it should give you a good start.

The next thing I would recommend is actually browsing the source of the default android launcher at the open source repo. You will need git to pull that down and you can find there here.

Read about using the AOSP here.