[iptables] ip_conntrack_* and ip_nat_* modules: explanation requested

I've googled and googled, but I can't find out which, if any, of these
modules I should load. More importantly, I can't work out exactly
what they do, or what the difference is between the conntrack and nat
modules. FTP and IRC are both working perfectly for me with only the
basic conntrack module loaded, so I can't work it out for myself.

Q1. Which, if any, of the ftp/irc modules should I load if I want
reliable client connections to an ftp/irc site?

Q1a. Why?

Q2. Do I need to load extra modules only if I'm running a ftp/irc
server?

Q2a. Why?

Please forgive my ignorance.

--
Peadar Ruadh mac Sheathain mhic Pheadair 'ic Mhata 'ic Pheadair