Roy, a propagator is a seed tray with a see-through lid, as above, a heated propagator is the same but with an electric heating element built into the tray to give the seedlings "bottom heat". You can also get thermostaticaly controlled heated mats to put under propagators. These are my heated propagators:
And many propogators are much larger than a seed tray - my greenhouse one is 4' x 2' 6" - homemade from waterproof ply, expanded polystyrene insulation, and plastic sheeting
Phil
