It works like this: Every time you complete an achievement, you earn achievement points. As your points score rises, you unlock new rewards for your account. Rewards so far include exclusive armor and weapon skins, permanent experience bonuses, raw currency, utility items, titles, and more.
The new achievements system is accompanied by a revamped UI panel designed to make it easier for players to track achievements and see where they stand. Oh, and the whole thing is retroactive, so achievements already completed will count towards your score total.
Account rewards are set to go live with the next Guild Wars 2 release.