Read the details in the article of Coinidol dot com, the world blockchain news outlet: https://coinidol.com/icos-airdrops-and-everything-you-should-know/
Actually there is a big difference between ICO and the airdrops even they taking the cryptospace by the storm,because ICO (initial coin offering) is an investment that needs of capital so that you can earn the percentage of your investing money and ICO project carried the signature of there product to promote into the market,while airdrops is only a free token given to those bitcoiners who having patronage there token.