Basically, an achievement is an unlockable something, unlocked by reaching a certain goal. That's a vague definition, so here are some example goals hinted at by what's leaked out:
- Reveal the whole map of Outland
- Do at least one daily quest a day for five days
- Get a character to level 80 on both factions
Achievements are clearly not yet implemented, even in the beta; many of the numbers in the screenshots are placeholder values, and the rewards for completing the achievements, whatever they may be, are completely unknown. It does look like there is a point system – the UI implies that each of the above achievements is rewarded with 50 points – but what might these points be used to purchase? Well, gear, most likely, but it's also possible that they may be a way to get prestige items, like special mounts or titles. Either way, I welcome yet another advancement path; more importantly, achievements are just plain fun!