Are there filters in drupal? I'm a wordpress user and I was wondering if Drupal has the ability to let coders modify the output, or return value of a function trough filters.

(in WP you can do this with add_action / add_filter)
Yes, there are a few methods.

One uses what Drupal considers filters.  I a not sure what the best starting point for these is, but the API reference for hook_filter_info should be a decent read.

Another is via preprocess and process functions.  Again, I am not sure what the best online starting point for these is, but take a look at the API reference and search for "process" and "preprocess".

The concept of Drupal 7 render trees along with field templates / template functions can also be thought of as a way to process content for display.

My copy is at work, but I think both filters and preprocess/process hooks are discussed in Drupal 7 Module Development.  I can't remember if render trees and field templates are in that book or their theming book.