That’s a more specific question.

From a user’s perspective, an airdrop is the reception of coins/tokens that an ICO decides to give away for free, normally (but not always) in the shape of an ERC20 token (that runs on the Ethereum Network). The quantities airdropped are normally small, and very often reach a measly market value if any, although one may be lucky and encounter one of those exceptional cases where it actually reaches a fair value.

The objective of the ICO is to promote itself by making it’s token known to the general public, in an intent (in general) to create buzz and generate potential prospect buyers of their token upon sale. The cost for the ICO is none, since the tokens are basically created out of thin air and not backed by anything of value.

On this forum there are multiple airdrop announced threads, although you may often encounter:

-   That an airdrop is not really an airdrop, but part of a campaign requiring you to perform certain (publicity) tasks. It is therefore not free in these cases, as it requires your time.

-   The airdrop may be a scam (as are many ICOs), and they are basically demanding your personal data for obscure purposes (i.e. email, name, etc.)

-   Some airdrops require KYC (Know Your Customer). This is theoretically to cover their backs legally, but in practice, one should really consider providing the information required, since it normally involves sending a photograph of your passport and such. Illegitimate use could lead to fraud by impersonating you (for a few normally worthless tokens after all).

-   More often than not, people look over their portafolio of airdropped tokens and see that month after month their aggregate values is close to just a few dozens of dollars (so don’t get your hopes up too high).