Faucets are websites, where generous (or not so generous) people give out BTC, most of the time in exchange for advertisements. Usually, you also have to solve Captchas in order to prove that you are not an automated bot. More often than not, these sites load pop-up windows with occasionally questionable content, but that’s how these sites make money. (I recommend a good anti-virus program!) Usually, you can never get more than a few Satoshis. A Satoshi is 0.00000001 BTC, or one hundred millionth of a BTC, the smallest BTC fraction the algorithm can currently deal with. Since the minimal amount of BTC in a transaction is currently 5430 Satoshis (0.00005430 BTC), these sites usually pay to another site, where the micropayments accumulate until your faucet earnings are over the minimum transaction limit. When you have exceeded the limit, these micropayment sites will pay out to your Bitcoin address.