I'm sorry because I've never known airdrop I'm still looking for information from this airdrop. For now I just follow bounty campaign only. Hopefully with a thread from me I can find the answer. 
Airdrop was simply a token giveaway from an ICO to attract more investors but most of it are scam, so as an investor you have to choose carefully and wisely but for us we can join at any airdrop since its free and all you need to do was register and fill up their form.