Achievements are one of two types of win. They tend to occur during play rather than completing it. Conditions of achievement usually come from the player rather than the game, and they do not result in defeat if they are not attained.
Some games monitor for achievements and congratulate the player on attaining them. Other achievements are purely personal, to do with creative expression (for example: Minecraft).
Achievements are thus more subtle, but also more fulfilling, than victories.