Well, it's a modern-themed shooter, but that's where similarities with CoD end.
It's mostly multiplayer. I mean, it has a single player campaign, but it's so bad that you don't even want to waste your time on that. In multi, it has a few game modes, each delivering a different twist to shooting people. In general, the most popular one is Conquest, where you have a few flags scattered all across the map. Controlling those flags is crucial to winning, as the more flags your team controls, the faster enemy tickets are depleted(apart from each ticket = one death). So it's mostly about territory and front line control - but don't expect people to really get that.
Other modes include CTF or Rush - an objective based mode, where one team attacks and other defends 2 M-COMS. After blowing those up, the map advances to another part with 2 MCOMs and so on, until attackers destroy all or defenders decrease attackers' tickets to 0. It is a really, really fun mode, but unfortunately Battlefield 3 maps are simply not suited well for it. Rush really shined in Battlefield:Bad Company 2, where the maps were designed to suit this kind of gameplay.
Another difference to CoD - and something that really makes BF games stand out from other modern shooters - are vehicles. You have tanks, jets, helicopters - all driveable and "shootable". Sometimes a few good pilots or tank drivers can change the outcome of a match, so it's good to practice using vehicles. Some maps are so vast that simply running around them is not really something you will want to do. So vehicle gameplay is definitely another layer to playing Battfefield games.
But the most important part is that this is a kind of shooter that focuses on
teamplay. While CoD usually concentrates on twitch-based, solo fragging, this kind of tactic will get you nowhere in BF. Sure, aim and movement are still important for you to survive in close combat, but the most satisfaction you will get out of the game will be when you team up with people that know what they're doing. There are different classes available - medic, engineer, scout, support - each with different weapons, special abilities and roles on the battlefield. Each team consists of 4 man squads and by performing different actions during the game you get extra points for doing them together with your teammates, even more boost when you do it with your squadmates. So it's really about strategy and supporting people around you, not only going in guns blazing or camping on a tower to farm frags.
It's sad that the game is unbalanced most of the time and many people don't really care about teamplay. But every once in a while you team up with friends or teammates that are really team-oriented and that's when you will get the most satisfaction out of the game and understand how good the whole concept is.
Hope that helps, if you need any advice or decide to go into BF4, let me know, we can play together
