Each WvW arena is made up of four huge maps. Each map is capable of holding more than 100 players from each of three factions (servers), which means that upwards of 1,200 concurrent players will be able to get in on the large-scale action. The beauty of having three teams duking it out is that things tend to balance themselves out naturally. If one team goes beast-mode and starts making life miserable for everybody, the other two teams can naturally gang up to fight against the first.
Although lore justification might be a little tricky, in a world with as much magitech the Guild Wars universe already plays host to, it's not too hard to imagine that people from alternate versions of the same world get spat into yet another mutually accessible copy of the world, where their first inclination is (naturally) to start beating the crud out of each other.
Said crud-beating is enabled by siege weapons: trebuchets, arrow carts, catapults, ballistae, and the inimitable siege golems. The construction of such weapons encourages team cooperation, as multiple players are needed to get any of these structures up and running. Keeps and towers have their own specialized defenses, so defending an objective can be just as fun as fighting to overtake it. WvW combat is all about keeping track of the moving pieces; you have to capture objectives to earn points, and then you have to continue defending them for strategic positioning, but you also have to keep supply caravans running so that you have the necessary materials to continue defense, and of course you always want to keep pressing forward to new objectives. Furthering the intrigue, NPCs populate these maps and can be convinced to turn mercenary and fight for -- or against! -- your team.
We've only skimmed the creamy surface of the information from the blog post, so if you're interested in further details, hit up the blog post to see more of what it's all about.