Running Functions within a Post/Page

Does anyone is there is a way to use php functions within a Post content without hard coding it into the template?? the reason I ask is I’m trying to do it for my first attempt at a plug-in. :-/

I know the nggallery does it with the following “tag ‘[nggallery id=1]'” but I cant seem to find anywhere that explain how to do it.

Any help is greatly appreciated

Thanks