Komplete Kontrol is't really an instrument itself, but more a utility used to browse your instruments (and the presets/patches made for those instruments). It can be used with one of the NI Komplete Kontrol keyboards to browse presets from the physical keyboard as well. It has other features, but these are the main ones.
Kontakt is an instrument, it's a sampler used to host many of the virtual instruments that you see advertised and discussed on this forum. Kontakt can run and be browsed through the Komplete Kontrol utility (as can the other NI instruments).
Hope that helps.