Polyjuice potion is a potion, that if you drink it, you can become the person who's hair is in the potion, for a short period of time. It is used several times, in the Harry Potter films/games. Harry and his friends use it several times, for their sneaky and cunning plans.
Polyjuice Potion is known for its horrendous taste which almost all the tasters vomit to. The taste is revealed to reflect the person whom the taster is trying to take the form of.
A single dose of Polyjuice potion will transform the taster for one hour, though larger doses keep you transformed for longer. It requires a month of brewing before Polyjuice is ready for tasting.
 Known Uses
In Harry Potter and the Chamber of Secrets, Hermoine reveals the Polyjuice Potion for the first time. She tells Harry and Ron that, in their search for the Slytherin Air, that there could be the possibility that Malfoy is said heir. To discover whether her suspicions are true or not, Hermoine produces a Polyjuice potion and has Ron and Harry become Crab and Goyle while she becomes a female in the Slytherin House. Once transformed, the three would invade the Slytherin House and talk Malfoy into telling the truth, all whilst being disguised as his friends.
Ironically, Hermoine accidentally picks her targetted girl's cat's hairs and is turned into a cat. She refuses to leave the stall that she hides in and tells the other two -- who turned into their desired targets -- to continue on with the plan.
In Harry Potter and the Deathly Hallows, Mad-Eye Moody gives several willing students of Polyjuice to transform them into Harry Potter. That way Death Eaters and even Voldemort himself wouldn't know which Harry was the real one. Despite Harry's objections to said plan, the group of kids continued to drink the potion.