Your bitcoins are in the wallet.dat file. If you lose it, you lose the bitcoins inside it.
From a practical point of view, it is correct that bitcoins are in the wallet.dat.
This is also the reason why replacing the wallet.dat with a previously backuped wallet.dat (from the other computer) gives you access to the bitcoins from the other computer.
From a technical point of view, it is
wrong that bitcoins are in the wallet.dat.
The bitcoins themselves reside in the bitcoin network.
This common misconception can lead to a massive security problem.
Short explanation:
The wallet.dat contains the private keys which enable anybody who knows these keys to spend the bitcoins associated with these keys,
now or in the future.
So regardless how few bitcoins you currently have in your wallet.dat (it might even be zero), if somebody gets access to it and copies it, this person will be any time in the future be able to spend the bitcoins associated to your wallet.dat.
It is the equivalent of making a copy of the keys to a high security vault which is empty, but someday in the future when the vault is full of wealth the persons which made the copy of the keys will be able to open the vault and empty it completely.
So make sure that right from the beginning you secure your wallet.dat by e.g. encryption of a freshly created wallet.dat.
Personally, I like the following article best:
https://bitcointalk.org/index.php?topic=17240.msg221715#msg221715
