There are great deteails and write-ups on Bitmex's website such as https://www.bitmex.com/app/perpetualContractsGuide https://www.bitmex.com/app/perpetualContra

There are great deteails and write-ups on Bitmex's website such as https://www.bitmex.com/app/perpetualContractsGuide https://www.bitmex.com/app/perpetualContractsGuide, but in general it is basically either futures or a type of swap contract, but all based in BTC.