There are two file formats used by MultiBit Classic to store wallets. Up until June 2012 (MultiBit Classic version 0.3.5) a format known as 'serialised' was used.
Starting with MultiBit Classic version 0.4.0 a new format, called 'protobuf' was introduced. Protobuf wallets are smaller, faster to load and easier to extend. They will be used in future MultiBit Classic versions to add more features such as wallet encryption.
Both wallet formats are supported in versions of MultiBit Classic from 0.4.0 and onwards. Older versions of MultiBit Classic cannot read the protobuf format. When you start MultiBit Classic there is a conversion utility that asks you whether you want to migrate older wallets to the new format. New wallets are always created in the protobuf format.
You can see which version of MultiBit Classic you are running by choosing the 'About MultiBit Classic' menu option. For Windows and Linux machines this is in the 'Help' menu and on Macs it is in the 'MultiBit Classic' application menu.