Hooks for Loading - GNU Emacs Lisp Reference Manual

You can ask for code to be executed if and when a particular library is loaded, by calling eval-after-load.

— Function: eval-after-load library form

This function arranges to evaluate form at the end of loading the library library, if and when library is loaded. If library is already loaded, it evaluates form right away.
If library is a string, it must exactly match the argument of load used to load the library. To get the proper results when an installed library is found by searching load-path, you should not include any directory names in library.
library can also be a feature (i.e. a symbol), in which case form is evaluated when (provide library) is called.
An error in form does not undo the load, but does prevent execution of the rest of form.

In general, well-designed Lisp programs should not use this feature. The clean and modular ways to interact with a Lisp library are (1) examine and set the library's variables (those which are meant for outside use), and (2) call the library's functions. If you wish to do (1), you can do it immediately—there is no need to wait for when the library is loaded. To do (2), you must load the library (preferably with require).

But it is OK to use eval-after-load in your personal customizations if you don't feel they must meet the design standards for programs meant for wider use.

— Variable: after-load-alist

This variable holds an alist of expressions to evaluate if and when particular libraries are loaded. Each element looks like this:
          (filename forms...)
     
The function load checks after-load-alist in order to implement eval-after-load.

15.10 Hooks for Loading