I dont mean this to sound like a stupid question, but... What enhances the graphic quality of a game or anything in 3D? Do they use something to make the graphics look more real, or does it just depend on the power and video card of the machine? I appreciate any help.

It depends, a powerfull graphics card is a plus but it takes more than just that. Textures and lighting are a big part of any realistic animation or game. Bad lighting can ruin the visual experience(for me). Soft Shadows, well designed characters and enviroments, fluid animations and so on give add to realism. You can have realtime soft shadows, HDR lighting and high resolution textures but kill all of it with a poorly modeled scenes.

modellers model things with 3d forms...the faster the machine and engine is-the more 3d forms they can use. therefore they can add more detail! also special effects such as explosions and lighting need speed too. so the faster the machine is -the more detail could be added and the more realistic the effect will look. ofcourse a good modeller can create good looking things with few forms (which is btw called polycount). and a bad modeller won't be able to make decent looking things with a large amount of polycount.
now if you still want to make highpoly things on a slow engine it will cost you fps (frames per second) which means your game won't be as smooth.