Crypto airdrops and ICOs are different concepts, even though they both involve new cryptocurrency projects. While airdrops don’t require any investment from participants, an ICO is a method of crowdfunding.
In an ICO, the project team conducts a token sale to collect funds from investors. ICOs started to become popular in 2014, when Ethereum performed a crowdfunding event to support its development. In 2017, the crypto space had an ICO boom, with hundreds of new projects adopting the method. If you want to learn more about ICOs, check out What Is an ICO (Initial Coin Offering)?