GAS is basicaly a petrol motor like in a chain saw or lawn mower, unless you have mega $$$ way to expencive for a trainer.

Electric is just that. an electric plane that runs on batteries, lots of options for an electric depending on how many $$ you want to spend every thing frome slow park fliers to full aerobatic and scale monsters again depends on the $$$

glow is an engine that runs on a fuel blended from methanol and oil, probably the most common and and widest range.

for a trainer most people start with a .46 size glow engine and a trainer plane to suit.