It is a full MMO, instead of levels it has EGO power, which functions in a similar way (One of Defiances downfalls is that this system isn't really explained to you in any way.) and I believe the cap is 10,000?
Buy to play as viral said, with nothing more than convenience or cosmetic items in the store. Paid expansions will be released in future as well.
It has a PvP side along with the PvE.
PvE consists of questing, story missions and co-op missions which are similar to dungeons. Whilst out in the world there are plenty of dynamic events such as protecting NPC's, stopping an ambush etc (Quite similar to GW2). There are also events known as Arkfalls, which consist of giant aliens attacking a certain area and needing big groups of players to destroy them, these remind me a lot of the invasions from Rift. You also receive a free vehicle at the start of the game as your main method of travel, the vehicles handle pretty well in my opinion and are quite fun to drive around with, especially using the boost to fly over ramps and hills.
PvP consists of modes such as Capture and Hold, Team Deathmatch and something called Shadow Wars (which appear to be PvP versions of Arkfalls) and these play out much like a normal 3rd person multiplayer shooter.
I certainly feel its worth a try, no subs of course so plays out much like a single player game with the added bonus of loads of other players to help. Feel free to add me in game anyway, my character name is Calvaria.
P.S. I should also point out the game is best played with a controller and very much a port of the console versions, although I would still recommend the PC version because the console versions look pretty bad.