At the core of the Bitcoin Core client is the wallet.dat file. This file is a BerkeleyDB database that functions as the brain of your Bitcoin wallet. It does not "contain" your actual Bitcoins—which exist on the public blockchain—but rather the required to spend them.
: In 2014, a notable security warning circulated on Reddit regarding how Dropbox wallet.dat files were being indexed by Google, making them searchable to anyone using "index of" search queries. indexofbitcoinwalletdat exclusive