Which part of my wallet should I NOT lose? .wallet file or private/public key? Simple question.  

I keep reading "don't lose your wallet", but which is the actual important part?  The .wallet file or the public/private key combo? Or is it both?
In the original Satoshi wallet (BitcoinQt), your private keys are stored in the wallet.dat file. SO essentially, you may not lose either.

You private keys are the data that grant you access to your coins, so those are the ones you should keep safe at any time. Most wallet clients call the file they store your keys in your "wallet". Various wallet apps will save the file under a different name, but mostly you can export it using the app itself or find it in their data directory.