What is the main difference in hardtail vs full suspension mountain bikes?

I mean obviously full suspension is going to offer a much more comfortable ride, but which is really more durable? Which is more likely to brake when going down hills, and hitting dirt jumps and stuff like that? Also what is your opinion on this bicycle? If you don't want to go to the link, I can just give you the info. It is a 2004, Trek 3700 hard tail. a href="http://atlanta.craigslist.org/sat/bik/2466638000.html" rel="nofollow"http://atlanta.craigslist.org/sat/bik/24…/a