Did you seriously read the link provided before?
the wallet.dat file contains your private keys (used to verify which bitcoins are yours), the actual bitcoin blocks are all over the network, the wallet.dat file just points out which ones you own...
Quote from the site above:
Locating BitCoin's data directory
The data directory is the location where Bitcoin's data files are stored, including the wallet data file.
[edit] Windows
Go to Start -> Run (or press WinKey+R) and run this:
explorer %APPDATA%\BitCoin
BitCoin's data folder will open. For most users, this is the following locations:
C:\Documents and Settings\YourUserName\Application data\BitCoin (XP)
C:\Users\YourUserName\Appdata\Roaming\BitCoin (Vista and 7)
"AppData" and "Application data" are hidden by default.
[edit] Linux
By default BitCoin will put its data here:
~/.bitcoin/
You need to do a "ls -a" to see directories that start with a dot.
If that's not it, you can do a search like this:
find / -name wallet.dat -print 2>/dev/null
[edit] Mac
By default BitCoin will put its data here:
~/Library/Application Support/Bitcoin/
[edit] Backup
The only file you need to back up is "wallet.dat". Ensure that BitCoin is closed, copy this file somewhere else, encrypt it, and put it somewhere safe. Ideally, you would put this file in two places: one nearby, and one 100+ miles away.
You can use the backupwallet JSON-RPC command to back up without shutting down Bitcoin.