shut down computers (or stop downloading music/porn/ect :P j/k)
a ping is how fast you can connect to the server/other consoles. the lower the better. if mutliple applications are downloading on a computer it can kill your ping.
if you are on cable internet, your connection speed can fluctuate (did i spell it right?) rise and fall. the more people on will slow you down. so afternoons and evenings you can get bad pings.
slower DSL can also be a problem. i cant play many games online because i have the cheapest DSL right now.
hope that helps..