How do I handle the risk that my hard disk crash? I have good back up system. Can my wallet be backed up?

Can I access it anywhere?

If so how do bit coin system know that I am the one accessing it?
Your wallet file contains the keys needed to sign transactions that transfer Bitcoins from your accounts. The system doesn't care who accesses what, it just cares that transactions are signed with the appropriate keys.

Back up your wallet file so you don't lose the keys you need to spend your Bitcoins. But remember, if someone gets their hands on any unencrypted copy of your wallet, they can spend your coins.