Bitcoin faucets are a reward system, in the form of a website or app, that dispenses rewards in the form of a satoshi, which is a hundredth of a millionth BTC, for visitors to claim in exchange for completing a captcha or task as described by the website. There are also faucets that dispense alternative cryptocurrencies.
Bitcoin faucets were developed by Gavin Andresen in 2010.
Rewards are dispensed at various predetermined intervals of time. Faucets usually give fractions of a bitcoin, but the amount will typically fluctuate according to the value of bitcoin. Typical payout per transaction is less than 1000 satoshi, although some faucets also have random larger rewards. To reduce mining fees, faucets normally save up these small individual payments in their own ledgers, which then add up to make a larger payment that is sent to a user's bitcoin address
As bitcoin transactions are irreversible and there are many faucets, they have become targets for hackers stealing the bitcoins.
Referral System
It is typical for faucets to have a referral system, where existing users referring new ones are rewarded with a pro rata portion of new users' earnings from the faucet. Unlike illegal pyramid schemes, earnings do not percolate to the top in the chain of referrals. The exact legal status of bitcoin faucets is unclear and can vary by jurisdiction, refer to legality of bitcoin by country.
