Roleplaying anthropomorphic animals - animals that have hands, walk on two legs, use tools, speak, etc. They don't necessarily have to be "furry", or human-shaped - birds, reptiles and the like are generally acceptable. The key is that they are intelligent enough to be considered a person. Even though this sounds like a cartoon in the making, this is not necessarily a light-hearted comedy - sophisticated dramas can be played out in this genre (think Watership Down).
More information