Synthpop = a type of music focusing on the synthesizer as the lead instrument. Usually considered softcore in the electronic world, synthpop is very light and bubbly; usually has other instruments involved like guitars and acoustic drums. Synthpop bands include Ladytron, Fischerspooner, The Teenagers, New Order, Soft Cell, Depeche Mode, The Knife, Owl City, La Roux.
Electropop= a heavier, more synthesized version of synthpop. Think synthpop is to rock 'n' roll as electropop is to heavy metal. Same instruments, different styles and speeds. Electropop is currently all the rage in the US and is heavily played on mainstream radio. Includes bands like: Jeffree Star, Blood on the Dance Floor, The Medic Droid, Breathe Carolina, LMFAO, Ke$ha, Lady Gaga, Adam Lambert.