I have seen two types of airdrops, maybe there are more, a new coin that decides to give the coins to people and forked coins that gives to you an equal amount of the airdropped coins than the ones you had in your wallet, both have some cons, in the first case if a condition is attached to the delivery of the coins then that is fishy, in the second case the airdropped coins are delivered fairly but most of the time the new coin wants to destroy the original and that is bad for your investment in that coin.