HI, if you want to see all processes, just type ps -uax into a terminal; if you want to see how much a process uses of system resources, run "top" in a terminal. There also is pstree to see process dependencies. If you run gnome, there a little application called "gnome system monitor" that can also show the processes (In KDE there is surely an equivalent).
which ones can I live without?
Well, depends on a lot of things you want (do you want a GUI? a firewall? network with DHCP? etc... And as for what every process does, you can try googling the name of the process you want to know more about.