But what happens when I have 101 transactions after I backed up the wallet.dat?
Bitcoin Core pre-generates 100 addresses for you to use as change address, new address etc. Everytime you unlock your wallet, your client refills the keypool. Lets say you back your wallet up at 0 transactions. If you use your wallet normally, you should have used up all your generated address at the point of backup as either the change address or a receive address. Hence, with your 101 transaction, Core may send the change to an address which might not be in your backup. Same goes for receiving Bitcoins.
What is the easiest way to have steadily access to my wallet and be safe in case of a hard disk crash for example.
How to backup the wallet properly? What data I need of the wallet to recover it any time?[/b]
If you have your Core as a HD wallet, ie having a HD symbol at the bottom left, you only have to back it up once. However, the seed does change with each password change. If you ever change your wallet password, you need a new back up.