The Medicine Cat is the cat who heals injured and sick cats.  Sometimes they get signs from Starclan in their dreams.  Their job is to interperate these dreams into meanings.  Each half-moon they visit the Moonstone (now the Moonpool) to meet up with the other Medicine Cats to dream closer to Starclan.  Sometimes the Medicine Cats have Apprentices.  They teach their Apprentices how to treat cats.  Medicine Cats serve their clan for their whole life as a Medicine Cat (unless they want to change).