Airdrop is not a coin, it is a way to get coins.
Several alt coins do it and it is a way to get coins without buying them.
Some are total for free and some you have to do some tasks.
If you want to get DeepOnion airdrops you have to be a jr. member in this forum an post addvertising in your signature bevor you can apply for the free coins.
If you just do it for the money it is not worth the time. If you want to support a project and are busy in the internet anway it is a nice way to get some extra coins.