Clipless- pedals without toe clips/cages/straps. Connect the pedals to your shoes directly with cleats.

Flats- platform pedals without any sort of attachment to the shoe, but may have spiky things for traction.

Personally, I'd be _scared_ to ride a MTB clipless! And I believe I'd fall just as much- for me it's practically never pedal-related. And I like my bike to have a nice soft landing on top of me! :-)

Nanci