Is every POAP an NFT? 

Is every NFT a POAP?

Every POAP is an NFT, which might have value in the future. Not Every NFT is a POAP as probabilistically NFT might already have  value while buying decision is being made. 

Although most of them end up being worthless just like POAP xD

POAP stands for proof of attendance. It's a specific instance of an NFT that you should be able to obtain for a limited time and perhaps only while at some location. The idea is to prove that you were there at the time.

all poaps are erc721 but not all nfts are erc721