ok Model B is 32KB ram (16kB for Model A) The Master series are a Model B+ (yes such a machine also exists), only put in a nice casing with 128KB ram by default and a number pad keyboard.
An Electron, from what little I know about them, is an attempt to make a Model B on a chip. Designed to be cheaper, so some hardware interfaces missing, and not 100% the same software/hardware wise as the original machine.
Games and software written to Acorn's recommendations will usually work on all three machines. There are games out there with different versions to best take advantage of each machine's hardware. For example Elite has Model B, Master, Sideways ram, and Second Processor versions.
Games: Camelot Exile