Flat pedals are basic platforms that only allow you to propel your bike using downward movement from your legs.
Toe Clip pedals have an extension from the pedal that wraps around the front end of your shoe, usually accompanied by a strap that can be adjusted to make a "cage". This allows your to use both upward and downward motion to propel your bike.
Clipless pedals are the most efficient pedal that allows the rider to use upward and downward motion to power the bike. It has a cleat "receiver" that allows a matching cleat attached to the bottom of the rider’s shoe to interlock with the pedal.