Pokemon Go is gearing up for another major system to be implemented in the form of a quest system. There will be two different types of quests: research quests that come from Pokestops, requiring players to complete tasks such as catching certain types of pokemon, and special quests that are story-driven and come directly from Professor Willow. Completing quests will earn you stamps. Get seven stamps and you can get extra items and an encounter with a legendary Pokemon. This system will also see the introduction of the game’s first mythical Pokemon, Mew. This will also complete the original 151.
Source: Game Spot