Baddeley and Hitch's Working Memory Model contains four systems: the central executive, which controls attention and higher executive functions; the visuo-spatial scratch pad, which stores spatial and visual information; the episodic buffer, a temporary storages system; and the phonological loop, or 'inner voice'. This model of memory explains our ability to carry out tasks such as mental arithmatic and chess - we store information briefly while actively processing it.
However, there is little research to explain some of these systems in depth. We are unclear on the exact workings of the central executive and some have critised it as just being part of the model because it 'explains the unexplainable'. Much of the evidence to support this model uses very artificial stimulus in artifical lab settings. This means it has low ecological validity and we may not be able to generalise it to situations outside of the lab.