user1173: What information does the Bitcoin wallet file used by the Bitcoin.org client contain? 
Is the owner's Bitcoin balance written in the wallet.dat file, or does it contain a list of transactions that have been made since the beginning of wallet creation?  Does the wallet's filesize grow bigger as more transactions are made?

user33: A default bitcoin wallet contains a list of private and pub key pairs. A number of keys are created on generation this is to allow for "swap" as you send money from it, these are not visible directly. This does not affect your balance which you use.
Your transactions are stored within the blockchain. If you generate new address to receive to then your wallet size will grow. If your wallet is moved to a new computer, with no blockchain, it will appear empty until you download blocks up until your transactions are known by the client.

user659: From looking at the code, the wallet.dat can contain entries of the following types:

name - an address book name
tx - a transaction
acentry - an accounting entry
key - a pair of public and private keys
mkey - a master key
ckey - an encrypted key
defaultkey - the default receiving address' key
pool - a keypool entry
version - the version of the software that wrote this wallet
minversion - the earliest version of the software that can read this wallet
cscript - a script
bestblock - a pointer to the end of the best blockchain seen
wkey - a private key (no longer used)
setting - used to store user interface settings (no longer used, except to hold an invalid setting to stop pre-0.4.0 versions of the client which don't understand minversion from loading wallets that are too new for them)

See also this answer.

1173: <p>What information does the Bitcoin wallet file used by the Bitcoin.org client contain? </p>

<p>Is the owner's Bitcoin balance written in the <code>wallet.dat</code> file, or does it contain a list of transactions that have been made since the beginning of wallet creation?  Does the wallet's filesize grow bigger as more transactions are made?</p>
 33: <p>A default bitcoin wallet contains a list of private and pub key pairs. A number of keys are created on generation this is to allow for "swap" as you send money from it, these are not visible directly. This does not affect your balance which you use.</p>

<p>Your transactions are stored within the blockchain. If you generate new address to receive to then your wallet size will grow. If your wallet is moved to a new computer, with no blockchain, it will appear empty until you download blocks up until your transactions are known by the client.</p>
 659: <p>From looking at the code, the wallet.dat can contain entries of the following types:</p>

<ul>
<li><code>name</code> - an address book name</li>
<li><code>tx</code> - a transaction</li>
<li><code>acentry</code> - an accounting entry</li>
<li><code>key</code> - a pair of public and private keys</li>
<li><code>mkey</code> - a master key</li>
<li><code>ckey</code> - an encrypted key</li>
<li><code>defaultkey</code> - the default receiving address' key</li>
<li><code>pool</code> - a keypool entry</li>
<li><code>version</code> - the version of the software that wrote this wallet</li>
<li><code>minversion</code> - the earliest version of the software that can read this wallet</li>
<li><code>cscript</code> - a script</li>
<li><code>bestblock</code> - a pointer to the end of the best blockchain seen</li>
<li><code>wkey</code> - a private key (no longer used)</li>
<li><code>setting</code> - used to store user interface settings (no longer used, except to hold an invalid setting to stop pre-0.4.0 versions of the client which don't understand <code>minversion</code> from loading wallets that are too new for them)</li>
</ul>

<p>See also <a href="https://bitcoin.stackexchange.com/a/854/659">this answer</a>.</p>
