Where I live in Portland, Oregon, the compost and recycling bins are picked up every week, but the trash bin only every second week. When the program first started (cutting back from weekly trash pickups), I was worried—how would composting suddenly reduce my trash output so much?
Well, it worked (mostly), and these days it's rare that I even fill the trash bin every two weeks. It turns out that a huge chunk of the trash I was creating is compostable, a lot is recyclable, and it has been an easy transition. But how does compost get made from the stuff I chuck into the green bin? In this video from KQED Science, we see how San Francisco creates their compost, and gives us an idea of why compost is so useful to plants. Have a look:
For a look at a different compost plant, here's a How It's Made segment on compost:
And if you want to make compost at home, there are a zillion videos explaining how.
(Via The Kid Should See This.)