Mr. Beanz is more or less right but I think that the use of the word "platform" seems to generally cause the confusion. Here is my understanding of the nomenclature:

Plain pedals: plain old flat, double-sided pedals. Like when you were a kid.

Platform pedals: pedals that are intended to be used exclusively with toe clips or straps. They are designed as one sided pedals.

Clipless pedals: Mr. Beanz post seems dead on.