One of God's (ghod's?) gifts to home repair.
It comes in many varieties, but all are designed to smooth and flatten walls. They are some sort of plasticky thing, starting out with the consistency of Cool Whip, but soon turns into the consistency of hard styrofoam.
It is used to smooth the joints between drywall (see Joint Compound), or repair holes, cracks, and irregularities in both drywall and plaster. It is typically applied with a putty knife.
- Get a dab of spackle on your putty knife.
- Apply spackle to wall near hole/crack/divot by running the putty knife at a low angle along the wall. Get a good hunk of spackle on the wall.
- Using the same putty knife, run the edge along the hole/crack/divot at a high angle. This removes much of the excess spackle, smoothing the surface. The smoother the surface, the less likely it is that you'll have to...
- sand the surface smooth. They sell special sandpaper-like mesh which won't clog as much with spackle. If the repair is small enough, or your putty knife technique is good enough, you may not have to sand at all.
If the hole in your wall is big, you have to get some sort of backing behind the hole to support the spackle. Your local home improvement warehouse will have all sorts of these and other fun items.
This is one of the easiest of the home repair/home improvement jobs, and you should be able to do it too. This saves lot's of money at the end of the year, when your college dormitory tries to make you pay for the nail holes you put in your walls.