It is many things, but started as more of an architecture for user-data interaction, where the data is in a computer system - and the user wants to interact with it (the user in user-model-view-controller unfortunately fell off at some point).

See:

http://heim.ifi.uio.no/~trygver/themes/mvc/mvc-index.html

As for where something like redis fits - I don't think it'd show up on the design that concerns mvc, no (it could be a cache inside model, inside controller - or even inside view.. Caching ui fragments for example?).