What is a Medusa and which groups have them?

A medusa is the jellyfish stage in the phylum of Cnidaria (jellyfish, anemones, corals). It swims free, in contrast to the anemone stage. All jellyfish have an anemone stage, and all anemones have a jellyfish stage, just less noticable than their main stage, either smaller or shorter lived.

Medusae are used to disperse the population, as it's free-swimming.