wallet.dat is the default wallet file for Bitcoin Core (formerly Bitcoin-Qt). Unlike lightweight or web wallets that outsource key management, Bitcoin Core stores everything locally. The file is an (since v22.0) or a Berkeley DB (BDB) file in older versions.

That power comes with responsibility. Treat wallet.dat like a stack of physical gold bars. Encrypt it. Back it up. Keep it offline when possible. And never, ever lose your passphrase.

: A pre-generated set of new addresses used for future change and receiving purposes. Default Storage Locations

pywallet is an open-source Python script that can extract keys from corrupted wallets. You will need Python installed. pywallet --dumpwallet --wallet /path/to/corrupt/wallet.dat

: The digital "signatures" needed to spend your Bitcoin.