What is the wallet in MultiBit? What is the actual wallet? The slight confusion comes from the install note -

"Backup all your wallets.

You can do this by exporting your private keys
using 'Tools | Export Private Keys'."


If a new wallet is created and called newwallet.wallet,
is that the wallet, the other file with a date string newwallet-2013xxxxxxxxxx.wallet, the exported private keys, or all of them?

Which one is to be backed up, or to be saved and imported into another system?

Also, are these multibit wallets only compatible with multibit, or can they be imported into other programs. On other programs they are called wallet.dat?
There is a section in the help which describes what each file is and the the data it contains. It is in the section 'More technical details' called 'File descriptions'.  

The wallet file (without the numbers) is the main wallet file. It contains your private keys and the relevant transactions. 

The wallet file with the numbers in the name is a rolling backup file. The numbers are a timestamp and it is 'the wallet that was saved just beforethe last save'.

The files with the suffix 'info' contain the labels for your addresses. 

Private key exports are typically given the extension 'key'.

You can backup your wallet either by :


Closing MultiBit and then copying the wallet and info files. 


Or


Exporting your private keys to a private key export file. This only contains private keys (no labels,  no transactions). You can import it into a new wallet and it regenerates the wallet (but currently you lose your address labels).


For higher value wallets I recommend backing up using both techniques and to two separate devices (just to be on the safe side).

Note that the MultiBit wallets are not deterministic so if you create more receiving addresses you will have new private keys created. You will want to redo your backups. You can choose how many new receiving addresses to create at a time (1, 5, 20 or 100) so I normally add more than I think I'll need for that wallet to save me the bother of re-backing up. 

The wallet format is in a format called protobuf. This is the format used in the bitcoinj library and is also used in the Schildbach Bitcoin Wallet. 

Developers can easily read and write this file format in other languages like C, C++, python though to my knowledge no-one has written parsers for the data format yet. 

Different clients tend to have import and export at the private key level (as that is the crucial data) and it's more standardised. The MultiBit private key export files can be imported directly into blockchain.info and the individual private keys can be transferred to and from anything that supports the WIF private key format. Pretty much all the clients import/ export this private key format.