To have an aggressive poker style means playing hands strongly by betting and raising; a passive style means you do more checking and calling. In general, an aggressive style is more effective at the poker table than a passive one. 
The reasons this is so, is because it gives you a bigger edge to win. When you bet or raise with a hand, the more likely your opponents will fold.
How aggressive your play is, depend on your particular demeanor; you may choose to play many hands and bet and raise with very weak cards or you may be much more selective about your starting hand but once involved play them stronger by betting and raising with every turn. This is the strategy used by many good poker players.
By playing passive poker, you can deceive them into thinking you hand is weak and they may bet anticipating you will fold your hand.
If you are playing a loose game where players call with weak hands, you will do better by going aggressive style. If you are playing against players who will only bet if they have good hands, you may want to bet your own hand.
By paying attention to how your opponents play you will have a better chance of determining what style to use against them.