The battle for Lake Wintergrasp is an attack and defend scenario, where the faction controlling the zone at the start of the game will have to prevent attackers from capturing the Titan Relic housed deep in Wintergrasp Fortress, inside Wintergrasp Keep. Players outside of the zone will know if they control Wintergrasp when they see the buff called Essence of Wintergrasp, which currently looks like the Replenishment buff. It can be a little confusing, but the cool folks at Blizzard made a new icon for it in Patch 3.0.8, so you won't miss it.
The Essence of Wintergrasp increases experience gained from killing monsters by 5% and allows players to collect Stone Keeper's Shards from 5-man dungeon bosses. Kind of like the underwhelming Spirit Shards of Auchindoun in The Burning Crusade, these Shards can be used to purchase various items from Jewelcrafting designs to a mammoth mount to heirloom gear. The buff also allows players to see Revenants, or greater elemental spirits guaranteed to drop cystallized elements, while they are in Wintergrasp.
Each game of Wintergrasp has a 40 minute timer. Defenders must protect Wintergrasp Fortress and prevent attackers from attuning the Titan Relic, a 10-second channeled action similar to capping the flag in Arathi Basin, until the timer expires. The Titan Relic is housed inside a room in Wintergrasp Fortress, and it is impossible for attackers to enter the keep unless they destroy walls, towers, or the Keep's door. The only way to destroy structures is with siege vehicles, which is one of the most fun things about Wintergrasp.
Siege vehicles are the heart of Wintergrasp PvP. Without them, a team on offense cannot win and a team on defense will have a pretty difficult time. All vehicles are capable of Siege damage, which is the only type of damage that can bring down structures, making them critical to success. Only players who have sufficient Rank can build a vehicle, although any player can pilot or ride one. Players can gain Rank by killing players or NPCs of the opposing faction. In Patch 3.0.8, all players start with the Recruit Rank, which allows players to move up in Rank and be eligible for Honor and Wintergrasp Marks of Honor at the end of the game.
As of the last iteration in the PTR, the Recruit buff becomes Corporal at five Honorable Kills, allowing players to build the most basic siege vehicle, the Catapult. With ten more kills afterwards, a player's Rank is moved up to First Lieutenant, allowing the building of all ground vehicles. Currently, there are three types of vehicles in the game -- the Catapult, Demolisher, and Siege Engine.
These vehicles are light, fast, and can carry one passenger. With only 30,000 hp, Catapults aren't very good at withstanding cannon fire, so are best used in numbers and with support from players on foot. These are sort of the Wintergrasp equivalent of zerglings. Catapults have two weapons at its disposal, the Plague Barrel, which has a cast time and a moderate range. Plague Barrels deal 500 siege damage to buildings and leaves an AoE cloud that damages players standing in it. It also has a channeled but usable while moving Flame Breath that deals about 1000 damage per tick to players. Catapults move at about the same speed as an epic land mount.
At First Lieutenant, or about fifteen kills in the new system (ten in the current one), players will be eligible to pilot a Wintergrasp Demolisher. These vehicles can carry up to three players, with one pilot and two seats, usually best for casters and ranged DPS. Passengers can cast spells and attack as though they were standing still.
The Demolisher has two attacks -- a very long range boulder throw that deals 1,000 siege damage to buildings, and a battering ram that deals about 1,500 siege damage. Both abilities can be spammed while the other is on cooldown, making quick work of walls or towers. Demolishers have 50,000 hp and move at about the same speed as a normal land mount.
Wintergrasp Siege Engine
These are the biggest, heaviest vehicles which players Ranked First Lieutenant can build. Siege Engines have only one attack, the Ram, which deals 2,000 siege damage and knocks back players as well as other vehicles. Another player must man the swiveling turret mounted atop the siege engine, a weapon that hurls a long range cannonball that deals 1,500 siege damage to buildings and a bit more to players.
Siege Engines also have room for two more passengers in addition to the pilot and gunner, although unlike the Demolisher, passengers may not cast spells or attack. Passenger seats on the Siege Engine are merely for protection, as opponents will be unable to target players inside a Siege Engine. It's a good idea to hop inside a Siege Engine when you're getting low on life or conversely, hop out when the vehicle is under attack. Siege Engines move at around running speed.