An autoflowering plant is one that will flower after a certain amount of TIME. Regardless of what the light cycle is doing. (Within sensible limits of course - don´t put them out in midwinter and expect a yield.) However they tend to be small and don´t yield a lot. A lot of people are of the opinion that if the penalty for growing, say, 10 plants is XXXXX, you might as well make them big ones.
basically most plants go through a vegitative state first then they start to flower(the buds) auto flower skips the veg state and goes straight to flowering so there usually done in 8 weeks. the best autoflower is a cross between lowryder and ak47 i think its called easyryder its made by the joint doctor