The wallet does not actually contain coins. Rather, it stores the keys that allow you to spend the coins that are associated with a specific Bitcoin address.
The actual value of coins in a wallet is determined by traversing the entireblockchain and summing up all credit/debit transactions values for the addresses stored in the wallet.
The wallet is not only a collection of keys. It also contains transactions, accounting information, settings and information about the blockchain.
bestblock: a pointer to the best blockchain seen. The client needs this information for a rescan after a restore of a wallet from a backup.
wkey: Public key (class CPubKey) followed by a wallet key (class CWalletKey). Apparently no longer used.
setting: used to store userinterface settings. It's no longer used. For backward compatibility, however, there is still one entry so that client with a version less than 0.4.0 can read a wallet.