The first thing which makes it unusual is that it's largely set outside in big, open spaces. Nestling amongst the impressive hills in these big open spaces are fairly big, impressive bases. These range from underground bunkers to rather impressive floating ones.
Now, with these spaces being so big, you're probably thinking that an interesting mode of transport would be useful, and you'd be right. You're equipped with a jetpack, with which you can soar to great heights and skim gracefully across the ground - whilst the power lasts. Then you have to trundle along on the ground for a bit until it's recharged.
Tribes is exclusively a multiplayer game; there are no single-player missions beyond training missions which can be completed in a matter of minutes. You can play it over the internet, but where Tribes really comes into its own is at a LAN Party.
The point of Tribes, you see, is to go up to the other team's base and capture/steal the flag from/destroy it. The other team will be trying very hard to do the same thing.
What makes Tribes so special, is... well, there are several things. Firstly, weapons. There are several inventory stations around most of the bases, from which you can obtain a whole range of weapons, different sets of armour, deployable gun turrets, cameras, remote sensors, mines... all the essentials of day-to-day life.
Secondly, vehicles. Most bases have a vehicle station, from which can be obtained various flying apparatuses with which to storm the enemy base. Which is, to say the least, fun.
Most importantly, however, Tribes is a *team* game. Let me give an example: a team of three people are trying to destroy key elements in the enemy base. Now, in most games of this type, they'd hurl themselves at the base as rapidly as possible, possibly forming into something of a team first.
In Tribes, on the other hand, two of your players would equip themselves with heavy armor and mortars, along with grenades and perhaps a few mines, whilst the third would equip with light armor and a laser targeter. The lightly armored bloke would comandeer a transport ship, which the heavy guys would board, and the whole lot would head toward the enemy base. The heavy guys can use their weapons in flight (the light one's driving), so they rain a rain of mortars down on the enemy until the base is reached, at which point they all jump out.
The lightly armored guy then uses his laser targeter to 'paint' targets for the mortar-carrying dudes, who then see exactly where to aim at to score a direct hit; in minutes, the base is in ruins.
On the other hand, whilst they were doing this, their base was being trashed by the other side because they didn't lay any mind fields or automatic turrets.
Winning a game of Tribes is all about cooperation, teamwork, weapons skill, tactics, and a fair amount of high-tech violence.
Graphics are fine; fairly pretty if you have a 3D card, probably not all that hot if you've not, but it'll be playable if you've got a fast (266+) PC. Sound is also fairly good.
Unfortunately, Tribes has a downside, and it's a major one. If you can't put up with laggy internet play, you'll need either a very fast internet connection, or a home LAN and *at least* three people to play with. Tribes cannot be played effectively with less than four people, and to be really great it needs at least six - my recommendation is that you get yourself a network card and get to as many Tribes-playing LAN parties as possible.
In summary... if you can get to a LAN party, or have a fast internet connection, Tribes is well worth getting. If not... find a few PC-owning mates and a few network cards, and you'll be find.