They can be called filters as well when talkin about PS. Plug-ins are lil programs that aren't a part of a main but expand what that program can do and can't work by themselves. You can add more on to Adobe if you wanted. I haven't done it yet tho, and usually i try and stay away from them and mess with layers as much as possible. Some of them can create some nice FX tho... like this one program called EYECANDY.
