Wintergrasp is a large PvP zone in Northrend (even on Normal servers) that sits between Dragonblight and Sholazar Basin. The zone's central feature in Wintergrasp Fortress, which can be under either Alliance of Horde control. A battle takes place roughly every few hours where the faction that currently controls the Keep tries to defend it against the opposing faction. At this time there is no limit to the number of players on a server that can jump into these sieges, so they are the largest (and subsequently, laggiest) PvP encounters in the game.
Wintergrasp was introduced to the game as part of the Wrath of the Lich King Expansion. Let's take a look at how it works.
Getting to Wintergrasp
As of patch 3.0.8 only characters of level 50 or higher can enter Wintergrasp. If you have the cold weather flying skill for Northrend, you can fly to Wintergrasp. Note that it is a no-fly zone, so once you are above it you get dismounted and parachute to the ground. The other way to get in is to use the portal that opens near the Battlemasters in Dalaran. It's not always available, but it does appear at the beginning of each battle.
Battles start 2.5 hours from when the last one ended, and can last up to 35 minutes.
The faction in control of Wintergrasp has access to a portal back to Dalaran. The other faction can use a flight point to leave the zone, or they can run to the perimeter and fly out.
Wintergrasp is an attack/defense scenario. One side tries to capture Wintergrasp Keep by busting through the walls and clicking on the artifact before time runs out, while the defending team tries to stop them.
When a match begins, defenders hold the 4 Towers at the corners of Wintergrasp Fortress, and attackers hold 3 Towers to south of the map. These Towers are defended with player-manned turrets, and attackers can earn bonus honor for destroying the Fortress Towers before capturing the Keep. Defenders can destroy the Towers to the south with vehicles. Not only does this transfer a special zone-wide buff from the attackers to the defenders, if all 3 Towers are destroyed, it also reduces the match time by 10 minutes.
As well as a Northrend-wide buff that lets you collect Stonekeeper Shards from PvE instances, the faction that controls Wintergrasp has access to the Vault of Archavon raid.
There are numerous daily PvP quests in Wintergrasp that reward honor points, gold, and Wintergrasp Marks of Honor. The quest-givers are in the Fortress for defenders, and near the starting graveyard for attackers. Marks of Honor can be exchanged for items at a vendor just east of the Keep door.
If one side in Wintergrasp is sufficiently outnumbered, they receive a buff called Tenacity, which increases health and damage output. This buff can stack several times depending on how bad the numerical imbalance between the two factions is. Subsequently, there is a possibility of encountering players in Wintergrasp that are invincible in 1-on-1 battles, and must be confronted by a group.
Both attackers and defenders have access to vehicles in Wintergrasp. They are the only practical way to destroy walls and towers, so they are of the utmost importance. There are six Goblin Workshops where vehicles can be acquired: 2 inside the Fortress for defenders, 2 at the south end of the map that belong to the attackers, 1 in the east that is neutral, and one in the west that is neutral. The neutral Workshops are captured by having enough players of your faction near the flag, like the towers in the Eye of the Storm Battleground. The more Workshops your team has, the more vehicles they can bring on the field.
A lot of the battle in Wintergrasp revolves around the east and west Workshops. If the attackers have both Workshops and are pressing on the Fortress with a full compliment of vehicles, it's very difficult to stop them. The defense really needs to take one or both of these Workshops to have a shot at winning.
As well as weapons and cannons, there are currently 3 types of vehicles available to players in Wintergrasp. To gain access to them, you need to earn rank by killing enemy players and guards.
Catapults - These are relatively small one-man vehicles which move at about the same speed as an epic land mount. They have a medium range Plague Barrel which damages structures and creates a toxic cloud that damages enemy players. Catapults also have a very short range Flame Breath for burning anyone who gets in their way. They don't have as many hit points as other vehicles, so they go down quite quickly when they come under fire.
Demolishers - The Demolisher holds one pilot and two passengers, who are able to cast certain spells from their seats. It throws boulders a long range and has a battering ram, both of which are good for smashing walls and buildings. Demolishers are slower than Catapults but they have more hit points.
Siege Engines - The heaviest vehicle players can build in Wintergrasp is the Siege Engine, which accommodates a driver, a gunner, and two passengers. Their main attack is a ram that does good damage against structures, as well as knocking players back. Siege Engines also have a long-range cannon manned by a player in the gunner position. Passengers on the Siege Engine aren't able to cast, but they can't be targetted either, so it's a fairly safe place to take cover when you're in trouble.
Although there were some flying vehicles in the beta test for the expansion, they were removed before release.
Rocket Propelled Goblin Grenade Launchers (RP-GGs) can be picked up near a friendly Goblin Workshop. These hand-held devices have a decent range and are well-suited for taking out enemy vehicles. You can carry 5 rounds, and can also use them while riding a Demolisher, which actually extends their range. The RP-GGs take a long time to fire, so hitting things with them takes a little practice.
Defence can man cannons on the walls and towers of Wintergrasp Fortress to bombard the attackers as they approach. The towers to the south of the map also have cannons on them.