Having a prehensile limb/tail means one that is capable of grasping objects. Lemur tails are sadly not, which means that they can't hang and swing between trees with the ease and grace that monkeys do.
The tail, however, can act as a way of communicating their location to other lemurs, similar to a sign post. Ring-tailed lemurs are often seen traveling with their iconic monochrome striped appendage pointing directly up towards the sky; just to make sure no-one gets lost on long journeys, especially the babies!
3. They are not evil.