The Palistinians are the aggressors. They were the ones who originally (in 1948) refused to accept the agreement to peacefully coexist.

They continue to target innocent men, women and children. They try to kill them. If Israel occasionally kills innocent people, it is because they are trying to kill terrorists, and they accidentally kill a bystander (often because terrorists keep innocent people around them as shields).

There is a recurring theme in a many of the other answers, which contains a major fallacy. People talk about the fact that israel has more advanced weaponry, and therefore they must be the agrressor. Aside from the fact that the palistinians have rockets which can blow up houses (unlike "fireworks"), this irrelevant. If you have a gun, and someone comes up to you and tries to stab you, are you the aggressor if you shoot him? Obviously not. There is nothing wrong with defending yourself with whatever you have at your disposal. If someone is coming to kill you and yours, you'd defend yourself as best you could.

People also talk about "peashooters" and slingshots," giving the impression of throwing small, virtually harmless pebbles. In reality, a sling can hurl a rock the size of an egg hard enough for it to shatter on impact. Think about what that would do if it hit a person's head. Dead is dead, whether from a rock or a bullet.

Oh, and one more point - if you want to know who the aggressor is, consider the following question: Do palistinians regularly walk through israeli cities without fear for their lives? Answer: yes. they complain when they are not allowed to go to work there! Do israelis go into arab towns? Answer: the ones that do often do not come back out.