Why does the moon have phases?

What are phases? Notice that when you look at the moon, it looks round on some days and on other days, it looks like a boomerbang or like a half of a pizza (yes i'm hungry). Those are, crudely explaining it, the phases of the moon.

Many people have the misconception that phases of the moon is due to the shadow of the earth falling on the moon. Actually, that's the reason for lunar eclipse not for the phases of the moon. In fact, the shadow of the earth doesn't fall on the moon so often or regularly.

So what's going on?

Again, this is a concept that's difficult to explain using words, so here's a site with diagrams to explain. Do read about it and teach others about the correct reasoning!