This thread should be pinned. Good read for beginners in crypto world. But where you said airdrop is given to people who in return do some task, is wrong or rather used to be wrong, I got cyder in airdrop and all they asked to do was fill in a form. No twitter retweeting / facebook likes etc. Nowadays things are changing and people mixing airdrop meaning with bounty.
It is an interesting question what is the difference between airdrop and bounty. Normally we cal it airdrop if it requires a single time action like join some social networks, post something, ask a question on telegram and etc. In most cases reward is fixed.(i.e. you will get X token for joining telegram). In bounty campaign you do tasks on weekly basis and report to bounty maanager. Most bounties use stakes system instead of fixed reward. But as always you can find airdrops with stakes or airdrops with lots of tasks. And there are bounties with fixed rewards as well. So it is all close...