I'll try to put it into simple words for you. An Airdrop is when you get free tokens of some project for actions like subscribing to their channels, making 1-2 reposts etc. This is a one-time action on the result of which you are given an amount of tokens that is usually worth about 20-30 dollars (of the ICO prices, the price on the exchanges might turn out to be very different). You don't need to own any eth, btc or whatsoever. Never give your money to those who claim they'll gie you a lot more tokens instead. They are probably scammers.
Bounty is more like a regular part-time job. You receive tokens (usually weekly) for regular posts and reposts in social networks, signature campaign on bitcointalk etc. Again, you give nothing but your work. They give you the money.
In general, you have to be a socially active person to participate in both, but the requirements for a bounty tend to be more strict than for an Airdrop (you'll need to have a specific number of followers on your profiles, for instance).
Note that many Airdrops and Bounties might value your work pretty instantly, but actually send you the tokens only after the ICO is finished (to avoid dumping of their token). That's what you mentioned by 'immediately sell it for btc'. People do that when they don't care about the project they work for and just want to get some money for their work. As many people are selling the token ASAP on exchanges, this can lead to price decrease and even more selling afterwards. But it's not supposed to be this way. The teams that allow you to participate in Airdrops and Bounties hope that you believe in their project and value their tokens.