'System' games (some of them use cards in addition to or instead of dice, and sometimes other things) basically are used to determine the limits of what a character can do. They help to give people roles in a role playing game - someone might be a healer, a warrior, a wizard - whatever, and the game provides various rules that help decide what a character can and cannot do.
That is the thematic idea. There are a lot of different systems, some focusing on specific genres. Dungeons and Dragons and Exalted are high fantasy, the World of Darkness is modern horror, Star Wars you are probably familiar with. Those are by far the most popular, though with the possible exception of 4e D&D they're rather complex.
Others focus instead on styles. "Big Eyes Small Mouth" is anime, while GURPS is more gritty-realistic.
If you're just starting, I would probably look for a 4e D&D or BESM game, and bug someone who is running one of those to show you how. They're both simple and quick. Most systems, as a rule, get out of your way pretty quickly outside of combat and magic.