Dark matter is pretty much exactly what it says on the tin: "matter" (or "stuff") that is "dark" (or "does not glow").  To be slightly more accurate: when we look up into the night sky, we see a bunch of colored dots of light: stars and planets.  If we look at pictures of outer space from telescopes, we see brightly colored galaxies and nebulae.  And so we assume that most of the matter in space is this normal, ordinary matter.  However, it is not.  Five times as much matter is actually "dark matter"—matter that neither reacts with nor gives off light.  This dark matter is instead detected by its gravitational force.  When we look at galaxies, we can measure the speed at which the non-dark matter is rotating; this speed is determined by gravitational forces.  We find that the stars on the outside of certain galaxies are rotating much faster than the visible mass of the galaxy would account for; we thus deduce that there is more "dark matter" in the galaxy that is exerting gravitational force but which we cannot see.  Methods like these are used to detect dark matter inferentially, as our telescopes are all light based (recall that x-rays and radio waves, among other things, are all different forms of light).

 

Along with dark matter, there is something even weirder and less well understood: so-called "dark energy."  Dark energy is a mysterious repulsive force, yet more prevalent than dark matter, that is pushing the universe apart and causing it to expand faster and faster.  Dark energy is in no way actually dark, and nothing about it is known as of yet.

 

As for uses for dark matter—probably not, but who knows?  We barely know what dark matter is (it could be "normal" matter that is merely dark, or it could be a strange form of matter as yet unobserved); even if we did know what it was and how to construct it, its properties do not lend themselves to many everyday uses.  We don't need, for instance, walls that are merely slabs of blackness—we have paint for that.  What dark matter's real use will be is in helping us unlock the secrets of the cosmos.

 

Research:

http://news.bbc.co.uk/2/hi/uk_news/wales/south_east/4288633.stm -- "Astronomers find star-less galaxy"

http://space.newscientist.com/article/dn14696-is-dark-matter-a-wimp-or-a-champ.html -- "Is dark matter a wimp or a champ?"

http://astro.berkeley.edu/~mwhite/darkmatter/essay.html -- "Dark matter"

http://physicsworld.com/cws/article/print/19419 -- "Dark energy"