What bitcoin data should I back up?
Since Electrum is a deterministic wallet having a copy of the seed is sufficient to get access to the coins. However if you also want to be able to restore the meta data in the wallet file such as the transaction and address labels and your contacts you need to periodically backup the wallet file too.

You can save a backup copy of your wallet file via file menu > save copy. Alternatively you can have your backup program grab the wallet file from the wallets directory.

If you're worried about your house burning down then do as @willtech says and keep a backup of your seed and/or wallet file in an off-site location. Cloud backups of your password protected wallet file are another option.

Note: Electrum doesn't store your wallet file, seed or private keys on any server.