A simple search on "Critical path" got this:
http://en.wikipedia.org/wiki/Critical_path_method[
^]
Why didn't you do that search yourself?
P.S.:
Sorry, on first glance, that linked seemed to describe an algoriithm, but apparently the topic is not about how to find the CP.
Anyway, digging further revealed this Wiki topic which does contain at least a couple of useful links and information:
http://en.wikipedia.org/wiki/Longest_path_problem[
^]