There is a thread on this in the FAQ.

Basically:

• Israel had more troops, and were able to increase their forces in size far more quickly and mobilize.

• Israel had better weaponry, and while Arab forces had better heavy guns and artillery, they had less shells to use them with.

• The Israeli army was generally better trained and better experienced than the various Arab armies. Many of the members of the Israeli army had fought in WWII under Allied command, plus there was a significant number of volunteers from other nations.

• The Arab nations fought as separate entities with no cooperation, unlike Israeli command, which was unified and very efficient relatively speaking.

• The Israelis were able to raise more funds for weapons, and were better at disciplining and training their army.

• The Israelis were able to make better use of the two ceasefires during the way for preparation, and their momentum/coordination was too much for the numerous (but chronically inefficient) Arab forces.