Module Enlightenment

Hello All,

I am relatively new to Drupal (Absolutely kick-a** cms in my opinion) and have a basic question regarding the development of modules. In many forum posts, articles etc. where someone is asking how to override built-in functionality such as adding or removing form fields or creating new content types without CCK, the response is often times "Write a custom module to do it". While this sounds fine and all, I may be missing the most basic piece of information. Would I need to create a separate module for each individual part of the core that I am trying to override or would I just have a single module (i.e. MyModule.module) that contains all of the various overrides for all forms or functions that I wish to change? I would assume that like a template.php file which contains all overrides at a theme level, an individual module would do the same thing but I'm not sure.

This may be a very basic and well understood part of module development but it has me stumped.

Thanks in advance for any help.