Passover basically is the retelling of the story of the Jews' exile from egypt, that is, their escape from slavery.
Although on the surface it may seem like a holiday of remembering the suffering that the Jews went through in Egypt, it's actually a celebration of what the Jewish people overcame with God's help and Moses' leadership.
It's marked with a very long evening dinner retelling the story of Exodus with songs, prayers and such.. It's a very fun night for alot of jews and happens once a year (usually falling around march-april, around easter). The synagogue service the following day is also fairly long but is well attended.
The celebration goes for about 8 days still by eating only unleavened bread or food without yeast, flour or wheat. This is so that the jews dont forget what their ancestors went through whilst escaping from egypt. The significance of the bread (or lack of) is that when the jews were escaping from egypt, they didnt have enough time for their bread to rise whilst cooking it, therefore they were left with uleavened bread for the next 8 days or so whilst journying to israel (or so the story goes). This is why you will see Jews not eating bread, cake, or anything else with flour in it over passover.
Hope this helped :)
My Own words btw-- Jewish atheist :P