Mealy and Moore machines are types of finite state machines. A Mealy machine's output depends on its present state and input, while a Moore machine's output depends only on its present state. Mealy machines can be converted to Moore machines by breaking states with multiple outputs into multiple states, and vice versa by combining states with the same output. Both machine types have advantages and uses, with Mealy machines being faster but more expensive, and Moore machines being simpler but slower.