Apt-get :: Remove libraries after install?

After installing the aps you want using apt-get, are the libraries downloaded to satisfy the install dependancies still needed for the ap to run? If not, is there an easy way to identify and remove unnecessary libraries?
Alternately, or in addition to this, could you back up these libraries to a cd, so you still have them somewhere in case you need them? And if you later do more apt-gets that require different libraries, you could add these to the cd as well?
I just figure I may as well get rid of them off my hard drive if they are no longer required in the day-to-day running of the machine?

original here.