Glazes can be laid on top of each other to create even more effects. This is called overglazing. Some "overglazes" are simply other glazes that are applied on top of another unfired glaze that will mature at the same temperature. True overglazes may also be applied after the base glaze has already been fired. These overglazes will require the ware to go through a third firing, at a lower temperature than the base glaze was fired to.
Slips and engobes are essentially the same thing. The difference in term is basically a difference in regional language preference. “Slip” is more common in Europe, and “engobe” is more common in North America. Both words refer to a liquid slurry consisting of clay or clay mixed with coloring agents. Slips and engobes are used to decorate wet greenware, adding color, texture, or two dimensional design. The advantages of using an engobe are that you can use them for raw (or single) firing, meaning you can apply them to work when it is still slightly damp or even leather hard. Unlike, glassy glazes, engobes usually produce a matt surface rather than a glossy shine. The exception to engobes being matt in texture is terra sigillata, which can be buffed to have a higher shine to it.