Both are, in general, used to control the area that's lit.
What are the relative advantages and disadvantages of both? When would you prefer one over the other?
In general snoots give more gradual falloff than grids. Grids are effectively a collection of very tight snoots next to each other, as a result they spill much less light at the end and give you a more direct narrower beam. They are used when you really need to control the light to highlight an object/detail and restrict it to a specific area.
In addition to differences in the quality of light, shoots are much easier to manufacture in an emergency from objects you happen to have to hand!