Faucet is just website which gives small amount of free btc/altcoin for solving captcha, viewing ad, playing game and etc. Their rewards are based on revenue from advertising networks and current price of coin.
Are they worth it. It depends on how you will use it. If you planning to claim from faucets all day long, it's not worth your time, because there many better paying jons. But if you want to use faucets on your free time, it's nothing bad to try it, but don't expect for a big earnings.
Only way to get decent earnings from faucets - hunting of referrals. If you will be able to find lot of referrals, then faucets will be worth your time.