Well... sütemény has a wider meaning than just "cake". Basically, almost any baked good can be called sütemény - cakes, bars, cookies, pastries (such as the delightfully named apácafing), even some salty or savory things like pretzels or pogácsa. And of course süti is just a cute diminutive version of sütemény.
But torta is much more specifically a cake (usually a multi-layered one).
Meg is a verbal prefix that doesn't have a very clear meaning, but mostly means that something is done to its completion:
- ért - to understand; megért - to comprehend, fully grasp
- mos - to wash; megmos - to cleanse (by washing)
- eszik - to eat; megeszik - to finish eating something, to eat it all
Meg is also used to give the action a perfective meaning, like with all other verbal prefixes.
Sütemény (don't forget the long 'é') is, well, baked goods in general, but more commonly used to refer to a cake. Specifically the type of cake that you first fully assemble and then put in the oven. Like a marble cake, or a fruit cake.
A torta is the type of cake where you just bake the sponge, and then assemble it and usually decorate it. Think of wedding cakes.