Effect: calms anxiety and soothes agitation
Rowling intentionally creates humor by contrasting the intended effect of this potion - peace - with the ingredient hellebore, which sounds rather evil (and is very poisonous as a matter of fact) and with the troubles the students have making it:
A plant sometimes added to potions (HBP22).
Cures the common cold.
If normal Muggle photographs are developed in certain potions, the resulting images will move.
"poly" Gr. many + juice
Effect: Transforms a person to look exactly like someone else. One dose lasts for one hour, but doses can be renewed, apparently indefinitely.
When the last ingredient is added, the potion froths a great deal and changes colour, but the resulting colour varies depending on the person supplying the last ingredient (CS12)
Polyjuice Potion can be used for cross-gender transformations. Crouch Jr. used Polyjuice Potion to impersonate his mother in order to escape from Azkaban, while she used it to impersonate him (GF35). Crabbe and Goyle appear to have used it periodically during their sixth year to pass themselves off as much younger, smaller girls in order to be less conspicuous (HBP21).
Judging from the Ministry of Magic leaflets on the subject, there appears to be no physical way of telling the drinker of Polyjuice Potion from the original. The only security measure recommended against Polyjuice Potion is to work out a prearranged method of verifying someone's identity, such as a password or a question that only the real person should be able to answer, but this system can of course be defeated by a sufficiently thorough and clever impersonator (HBP3).
The juice may be used in Strengthening Solution at some stage (see) and is definitely a potions ingredient.
The quills are used in boil-cure potion (see).
The eyes are used in Swelling Solution (see).