Just wanna add to DJuxtaposition's description (without the 'graphophile blasting' of course :) )

For those who didn't play/know tribes 1, tribes is a first person shooter. But, instead of being focused on the individual as quake and UT type games are, its extremely team based. Most gameplay types (CTF, defend and destroy) require a team to work together in order to win.
Oh, and you can just read: Tribes

Some of the improvements over Tribes 1:
Water
More packs (item gotten at an inventory station that, when activated, performs a function (ex: sheild, repair, etc))
Land and Air vehicles (T1 only had air)
Various types of grenades (concussion, whiteout, etc)
New Weapons (rocket launcher, shocklance)
New gameplay types (Siege(think UT's assault))
Bots (some people don't like them, but they are there)