Filename            | Description
--------------------|----------------------------------------------------------------------------------------------------------------------------
banlist.dat         | stores the IPs/Subnets of banned nodes
bitcoin.conf        | contains configuration settings for bitcoind or bitcoin-qt
bitcoind.pid        | stores the process id of bitcoind while running
blocks/blk000??.dat | block data (custom, 128 MiB per file); since 0.8.0
blocks/rev000??.dat | block undo data (custom); since 0.8.0 (format changed since pre-0.8)
blocks/index/*      | block index (LevelDB); since 0.8.0
chainstate/*        | blockchain state database (LevelDB); since 0.8.0
database/*          | BDB database environment; only used for wallet since 0.8.0; moved to wallets/ directory on new installs since 0.16.0
db.log              | wallet database log file; moved to wallets/ directory on new installs since 0.16.0
debug.log           | contains debug information and general logging generated by bitcoind or bitcoin-qt
fee_estimates.dat   | stores statistics used to estimate minimum transaction fees and priorities required for confirmation; since 0.10.0
indexes/txindex/*   | optional transaction index database (LevelDB); since 0.17.0
mempool.dat         | dump of the mempool's transactions; since 0.14.0
peers.dat           | peer IP address database (custom format); since 0.7.0
wallet.dat          | personal wallet (BDB) with keys and transactions; moved to wallets/ directory on new installs since 0.16.0
wallets/database/*  | BDB database environment; used for wallets since 0.16.0
wallets/db.log      | wallet database log file; since 0.16.0
wallets/wallet.dat  | personal wallet (BDB) with keys and transactions; since 0.16.0
.cookie             | session RPC authentication cookie (written at start when cookie authentication is used, deleted on shutdown): since 0.12.0
onion_private_key   | cached Tor hidden service private key for `-listenonion`: since 0.12.0
guisettings.ini.bak | backup of former GUI settings after `-resetguisettings` is used

Only used in pre-0.8.0
---------------------
* blktree/*; block chain index (LevelDB); since pre-0.8, replaced by blocks/index/* in 0.8.0
* coins/*; unspent transaction output database (LevelDB); since pre-0.8, replaced by chainstate/* in 0.8.0

Only used before 0.8.0
---------------------
* blkindex.dat: block chain index database (BDB); replaced by {chainstate/*,blocks/index/*,blocks/rev000??.dat} in 0.8.0
* blk000?.dat: block data (custom, 2 GiB per file); replaced by blocks/blk000??.dat in 0.8.0

Only used before 0.7.0
---------------------
* addr.dat: peer IP address database (BDB); replaced by peers.dat in 0.7.0