In the client (and most of its derivatives), wallet.dat is a Berkeley DB (BDB) file that stores:
In the early days of Bitcoin—when a pizza cost 10,000 BTC and “mining” meant running software on a laptop in your dorm room—the humble wallet.dat file was little more than a digital curiosity. A few kilobytes of encrypted randomness. A key to nothing. indexofbitcoinwalletdat
Misconfigured servers may allow "Directory Indexing," where a browser displays a list of all files in a folder (often titled "Index of /..."). In the client (and most of its derivatives), wallet