Player vs. Player combat in MMOs takes on many forms. Sometimes its small groups of opposing players fighting in a small arena, like in World of Warcraft, objective based warzones like in Star Wars: The Old Republic, or it's on nearly the entire time, like with Eve Online. With the The Elder Scrolls Online, ZeniMax Online has decided to go with something potentially epic in scale, though ultimately requires a great deal of player involvement.
PvP takes place in Cyrodiil, a triangular shaped region with multiple castles and locations evenly spread out across the map, and a homebase for each faction located in the corners. The most important thing to note here is that Cyrodiil is big. Really big. Traveling anywhere by foot can take a long time, and you might want to wait until you've been able to invest in a horse before jumping in. There is fast travel between friendly locations via a wayshrine system, but you still should expect to hike your way around the map. Patience is definitely needed, especially if you're the type of PvP player who prefers jumping right into the action.
Cyrodiil is definitely designed for players to experience while in a group or guild, so if you're not the social type, you're probably going to feel left out. Going off by yourself or attempting to capture any of the objectives solo is tantamount to suicide, as all of the hostile NPCs you'll encounter are maxed out at level 50. You won't have much fun if you jump into Cyrodiil at the minimum level (10), and you'll likely last about 10 seconds should you decide to pick a fight with one of the neutral Imperial camps or try to take on a patrolling NPC from a rival faction on your own. Higher level characters may be able to take down one or two NPCs without much problem, but much like other team-based MMOs, it's almost a requirement that you stick with other players. Plus, it's much more satisfying to take down a squad of high-level NPCs with a mixed group of other people than trying to take them on alone.
While there are various bounty boards that offer minor quests like killing a set number of enemy players or scouting out a location, the primary goal of PVP in The Elder Scrolls Online is to capture and secure the various keeps that dot the map. Surprisingly, there's a good deal of strategy involved in conquering or defending a keep beyond just capturing a flag or control point. All of the keeps are surrounded by three additional locations; a lumber mill, a farm, and a mining site, each of which provide bonuses based on how long they've been under your (or your enemy's) control. For example, Lumber mills help strengthen nearby walls and doors and can even auto-repair them over time, while farms can boost the armor and health of friendly NPCs in the area. If you're on the offensive trying to capture a castle, it's an important strategy to capture these areas first to deprive the enemy of bonuses or reinforcements.
Taking on castles themselves is a long and involved process that requires a great deal of effort from the players involved, and can actually be a dull experience - at least at first. To even get in the castle, you have to break down one of its gates or walls, and this is where player-operated siege weapons play an important role. Taking the form of trebuchets, ballistas and battering rams, you can purchase siege weapons and equipment from special merchants located in each friendly keep, costing you either gold or the currency you earn from competing in PvP. All of the siege gear acts like a consumable (that you can thankfully pick back up if you need to move), and there's a handy indicator that appears on the ground to show where you can deploy them. Using any of the siege weapons is surprisingly easy, as you can simply use your mouse to aim and there's guidelines that show where your projectile will land. But at the same time, it's not terribly engaging to click on a target, fire, and then wait for your catapult to reload over and over.
When you take a step back and look at it from a distance, it's can be quite fascinating to watch an ongoing siege, and see players operating a dozen different trebuchets all chucking fiery projectiles at a castle gate, while enemy players try to knock them out with siege weapons of their own from along the castle ramparts. But whittling down doors can take a long time, and if you're not manning siege weapon or trying to snipe enemy players with ranged attacks, it's not the most adrenaline pumping experiencing. Once the doors are down, however, then the pacing of the game can pick up, as melee-based characters can get the opportunity to get into combat with any enemy defenders inside.
The PVP events I participated in were limited - I only encountered about a dozen enemy players throughout my several hours of play, and the group of friendly players I joined up with only numbered around 30 or so. Despite being a mixed bag of waiting long stretches of time followed frantic close quarters combat, I did enjoy some of my time in combat with other players and conquering keeps, but I probably would've had more fun if the game had scaled my character up to a level on par with the NPCs. Regardless of its current state, however, you can definitely see the potential TESO has for some really enjoyable PvP action once there are hundreds of players all vying for control of Cyrodiil and battling each other in large scale battles. If the idea of fantasy battles on a massive scale piques your interest, you'll probably want to keep an eye out to see how TESO's PvP evolves once the game hits launch.