For both the QT and UI wallets, the file wallet.dat contains your private keys which grants access to your coins on the blockchain. If this file becomes corrupted or lost you will no longer be able to access your coins without restoring a valid backup.
It is best to store your backed up wallet.dat somewhere other than the computer you normally use to run your wallet. SD cards and thumb drives are ideal candidates.
Because all NIX wallets utilize HD key generation, you do not need to backup your wallet after generating 100 or 1000 receive addresses. All receive addresses you generate are based off of a single master private key which will grant you access to all future generated addresses.
When encrypting your wallet.dat file, a new master key is generated and used from then on. This prevents anybody who may gain access to your previously unencrypted wallet.dat file from gaining control of addresses you've generated after encryption. Because of this, encrypting your wallet will require you to create a new backup of your wallet.dat. You should also delete any previously created backups that are not encrypted.
You can manually copy your wallet.dat file to a new location by navigating to the NIX data directory and making a copy of the wallet.dat. The default location for this file is inside the wallets directory if it exists, otherwise, the root data directory (nix).
You can backup your wallet.dat file from within the running nix-qt application by selecting "Backup Wallet..." from the File menu.
You will then be presented with a standard "Save file as" window where you can select a location and name for your backup file.