Tera is a fantasy action MMO, which differs from classics like World of Warcraft by adding dynamic, skill based combat.
Traditionally, fighting in online multiplayer role playing games is based on statistics - you gain experience throughout the game which makes you stronger, allowing you to beat tougher opponents. In Tera, while there is still experience levelling, you can in basically exceed that if you're good at combat. Fighting in Tera looks really fun and fast paced.
Graphically, Tera's environments are really stunning. It's a brightly coloured world, and richly detailed with impressive scale. If you have a powerful PC, this could be the most impressive looking MMO you've seen.
With any MMO, the biggest worries are story and quests. The story might be generic or almost non existent, and the game might be full of 'grinding', lots of mindless 'kill twenty boar' type missions.
In terms of story, Tera's interest is based on its political system. You have to make a guild with other players, and challenge for political office, 'Vanarch'. This can be done through battle with other wannabe Vanarchs, or by gaining the votes of other Tera players. The beta phase has exposed grinding quests, but the fun combat makes this much less boring.
Tera is a great looking MMORPG with a lot of potential. It will be interesting to see if it has what it takes to survive next to the big titles like World of Warcraft.