The enigmatic kitten named Nyan Cat became a cryptocurrency formerly in January 2014. It could have been the competition for Dogecoin but sadly it could not be.
Nyancoin is a monetary system. Like other currencies, its value can decrease and increase at any time. It can be exchanged for goods and services, passed from one person to another, and generally used in the same way as any other currency.
The Wallet software is a more convenient option as it can create Nyancoin addresses automatically, store your private keys so you don't have to enter them each time, display your current balance, and a list of transactions you have made.
However, like individual addresses and keys, you should back up your wallet regularly. If you don't, you could lose all your Nyancoins permanently.
It is rumored that the original developers left the project mysteriously, with no reason to explain why they became Doges or something they say.
The original website and the other services stopped working after all that, but all was not lost, as enough people kept the software afloat running and continued mining the coin.