I only know a little about tribes, but from what I understand it very straight up death matchy.  PS is a world away from that.  It's a bit hard to describe in a forum post, so I'll just post the page from the Wikipedia page:

Gameplay

At any one time, the PlanetSide server hosts potentially
thousands of players in a variety of landscapes, from the war on foot to
the air, across desert, mountains, forests and swamps. The goal is
simple: each faction must attempt to capture as many facilities as
possible on the surface of the planet while denying them to their
enemies. There are many strategies and opportunities that skilled
players can utilize in order to direct combat operations. The player
controls his or her chosen character from a first-person perspective. This gives combat a more direct role and emphasizes skill.

Character progression

While a first person shooter at heart, PlanetSide does have an
experience/leveling system. Earned experience is divided into three
categories: Battle Experience, Support Experience, and Command
Experience. Battle Experience is gained from elimination of enemy
soldiers, the capture of base facilities, and exploring and interacting
with the game world. Support Experience is earned through "Assist"
kills: after aiding another player, the first player gains a percentage
of experience the other player gains through kills. Command Experience
is gained from leading a squad or platoon in a successful base capture.
Functionally, Battle Experience and Support Experience are identical,
both contributing towards Battle Rank, which leads to benefits such as
the ability to use different equipment, implants, or appearance
upgrades. Accumulated Command Experience lets the player use several
team-oriented abilities. Currently, the maximum Battle Rank is set to
40, while the maximum Command Rank is set to 5. Players who attain
Battle Rank 40 are granted every certification in the game.

Most Battle Ranks give you a certification point which can then be
spent on certifications, while each Command Rank unlocks a special
ability accessed via an item called a "Command Uplink Device (CUD)".
Certifications can be used to unlock new weapons or vehicles you can use
on the battlefield, thus enhancing a character's flexibility.

[edit] Vehicles

PlanetSide contains 38 vehicles, ranging from interceptor aircraft to battle tanks to Battle Frames (introduced with PlanetSide: Aftershock).
No one player is able to access all 38 vehicles normally, as many of
them are empire-specific meaning that only members of a certain empire
can acquire them via a vehicle terminal. Players that have invested
certification points in the Hacking tree are able to "jack" enemy
vehicles for their own empire's use. Some staple and basic vehicles are
available as Common Pool vehicles, meaning that any soldier can acquire
them, assuming they have the proper certification to do so. Vehicles
play a vital role in PlanetSide, and an experienced pilot or gunner of even one strategic vehicle can often turn the tide of a battle.

Organization

While a player can experience the game solo, the best way to succeed in PlanetSide
is to join a squad. A squad is an ad hoc group of up to ten players led
by a squad leader. Two or three squads can form a platoon, for a
maximum group of thirty players. Benefits of being in a squad include
the sharing of experience (resulting in a speedy rise in Battle Rank)
and the results gained from working as a team.

Like most online games, a player can create or join a clan, which is called an Outfit in PlanetSide.
Whereas a squad is a temporary group, an outfit persists even if all
members are off-line. Outfits are managed through a panel in-game,
allowing players to be promoted/demoted. Outfits have eight possible
ranks, with the top four being officer ranks, allowing members to invite
and kick players to and from the outfit. Outfits can range in
membership from two (the minimum number required to form an outfit is
ten players) to an infinite number, the largest outfits have over 400
members. Sometimes these large outfits recruit by inviting random
players, generally causing the outfit to have a poor reputation. The
largest outfits in the history of the game have approached and sometimes
exceeded 1000 members. In 2006 an "Outfit Wars" event was launched for
competition solely between outfits on each server.

"Reads a bit like a persistent global BF3 Conquest match."

In the simplest terms, yes.  But imagine a BF3 conquest match on a persistant world with hundreds of players on each three factions fighting at once across a whole continant.  Then add vehicles which require crewing, not the 'one person can operate a a whole tank' but a tank requires a driver, and then a gunner for each of the turrets; war walkers which require coordination you would enevr see in BF3, but whose practice pays off to the point they can win battles;  and a play area so vast drop ships are needed to ferry squads around, and are.

15 minute dev presentation video:  http://www.tentonhammer.com/video/preview/planetside2?utm_source=Reloading...&utm_campaign=6b30b666a0-Reloading_03_02_2012&utm_medium=email

Wow. That looks fucking amazing!

Meh, 3 minutes cheery picked from the full 40 minute presentation.  The full version was much moer mind blwoing IMO.