Airdrops only provide or need minimal requirements or instructions. It is just a way of spreading the coins through minimal promotion or advertisement. Bounties need a lot of work to do, such as a long-term promotion of the coin in social media like twitter, facebook, youtube, blogging, and even here in this forum. They give a relatively higher amount of reward, in the form of their coins or currency that can be exchanged in the future once the campaigns ended. It is very hard to find good finds on airdrops, and to earn more you need to go do bounties.