How do they differ and what are their uses?
Typically slips have been used to cover a less expensive clay body with one which is more attractive or has color characteristics that the underlying body does not. It may be that the underlying body is easy to throw with but a white surface is wanted for decoration or a black surface is wanted which would be almost impossible or expensive to get in a body. The use of slips have moved far beyond that now. Defloculated clay bodies can be quite fluid while having very little water in them. They can be stiff enough to hold their shape and give a surface that would be almost impossible to find just by throwing. Steven Hill, http://www.stevenhillpottery.com/, is well known for this technique. Another decorative use of slips is trailing difference colored slips on a pot and then with a tool such as a feather, creating a design. Ceramic Arts Daily shows this technique.
Engobes are very similar to slips but they will have fluxes and colorants in the recipe. They can be formulated to fit on clays from the range of wet to those that have been bisque fired. Underglazes are basically engobes. Possibly the distinction between the two is that one assumes a clear or transparent glaze will be put over the underglaze while an engobe may or may not have glaze applied.