How to detect what file was installed?

[houkensjtu <houkensjtu@gmail.com>]

Hi debianer!
I am rather new to linux and debian.
Here is my "stupid" question:
Usually I use aptitude to search for a package, and install and uninstall packages. My problem is, I can't figure out what files apt copied into my computer, and where were those files installed. 
I know I can use the "which" command to detect where a executable file reside in, but how about all the other stuff? 

Further, if I am not using apt but compile a software from source code, what is the proper way to find out what/where did the installation copied to my system?

Thx in advance!