Airdrop is a program where you can get any token or coin from developer, after joining a social media account, or share event and from reff,, bounty as the same as Airdrop, different at the collection of the token or coin that you must do several task and get verified at the end of the task,, you can get from reff to, selfdrop has the same objection with Airdrop but you just send a small amount to zero of token or coin to addresser so you can get excange with another coin or token