Fedimint is a federation ecash system (and can be much more since it's modular). Cashu is a non-federated ecash system. Both take in bitcoin and issue ecash notes. These ecash notes are 1:1 IOUs for bitcoin but use blinded signatures by the mint to preserve privacy. They also use LN for interoperability between other bitcoin projects/wallets/etc.
happy to go further if this wasn't detailed enough.
also check out https://fedimint.org