Once upon a time a young man living in the San Francisco area had a computer. His name was Craig Newmark and he started posting things about cool things going on around town. Its popularity grew in leaps and bounds, so Craig had to get a Listserver majordomo, which required a name. He wanted to call his list 'sf-events' but was persuaded to call it Craigslist, since that is essentially what it was at the time.
Later, people started adding other things to the list, like apartments for rent and garage sale items. Craigslist is in every state and in most countries and it's become a great way to sell things and services to people.
If you have bunk beds that your children have outgrown, post them on Craigslist and watch the buyers come in droves. People are always searching for great deals and there are tons of products priced quite reasonably there. Some people scour garage sales in their neighborhoods, purchasing bargains and then reselling them for a profit. Craigslist is another great way to earn extra money online.