What is an airdrop?
Some cryptocurrencies provide you with a reward when you achieve a defined goal. Airdrops can paid as a one-off occurrence or as a regular occurrence.
Usually the goal is holding of the primary cryptocurrency for a set duration (e.g. 3 months) and the reward is payment in a secondary cryptocurrency.
Airdrops among the larger cryptocurrencies are becoming less common.
MCO, NEO and PAY (now TENX) are all leading cryptocurrencies that offered airdrops early on in their lifecycle in order to drive user adoption. As their value and user base increased, they ceased the airdrops.
Airdrops to avoid
If you search Google for 'cryptocurrency airdrops' you will see a list of directory sites that provide details on cryptocurrencies that have airdrops. While it might be tempting to use these sites to find and invest in cryptocurrencies that promise a good level or return, I would urge caution. Do your research on any cryptocurrency that you are thinking of investing in - see how popular it is, look at its market cap, 24 hour volume, purpose, how actively it is being developed, etc.