The name practically says it all. If there's matter, then antimatter must be the opposite of matter. Which is what it essentially is, but why not go more into what it really is.

In order to name a particle of antimatter, you basically add the prefix "anti-" in front of a subatomic particle to change it. A proton becomes an antiproton, an electron becomes an antielectron (also known as a positron), neutron becomes antineutron, etc. The structure of anti-matter is exactly the same, except antimatter is a mirror opposite.
external image 525px-Quark_structure_proton.svg.png This is the structure of a proton, with the two up-quarks and one down quark. Do not be confused with charges, as it is not such that Matter has a positive charge, and antimatter has a negative charge, but that antimatter is opposite in every single way, but not structure as this next picture shows:

untitled.JPGThe structure of the proton is exactly the same. Antimatter is represented by the chemical symbol of a bar placed above the name of the matter particle. For example, a proton would be p and an antiproton would be p

Antimatter reacts violently with normal matter, in a reaction called anihilation. When antimatter meets mater, both cancel each other out and send enormus amounts of energy in the form of light and heat everywhere.

Creation of Antimatter:
Very little is known about antimatter and how it is created in the universe, but scientists here on Earth have discovered ways to create particle of antimatter, and even a few atoms of antihydrogen (1 antiproton, 1 antineutron, 1 antielectron) in very precise conditions. Since everything around us is made of matter, containing antimatter becomes amazingly difficult. A device has been created called a Penning trap that uses a field of electrically charged gas that automatically repel any antimatter that comes towards it, making the antimatter "levitate" in the middle of the trap.

The LHC (Large Hadron Collider) was used to study antimatter. The scientists at CERN are now producing antimatter at a fairly steady rate, but the energy involved to create antimatter creates a price tag on antimatter. This price tag is at about $62,5oo,ooo,ooo,ooo per gram of antimatter, this is based on the fact that a picogram (.000000000001 grams) costs about $62 million to create, and which is what the CERN laboratories currently produce per year.

Uses of Antimatter:

So, great. It costs 62 Billion dollars to create a gram of antimatter. That's more money than Bill Gates has ever had, he currently has about $56 bilion. What exactly can we do with a gram of antimatter? Well, remember back when you saw that antimatter annihilates with matter? Well, the energy created is a very large amount, and most of it is sent to other particles, basically rubbing them (friction) and heating them up. Imagine a sphere of white light just appear out of nowhere, that is what an annihilation would look like. 1 gram of antimatter, however, would create a sphere of destruction about the size of the 5 boroughs of NYC. Now don't think atomic bombs, where there is radioactive fallout everywhere, and buildings are blown apart. Just imagine that after an annihilation of this type, there is NOTHING. Air, water, cars, people; everything within the blast radius gets annihilated just as the antimatter and matter. No radioactive fallout, just a perfect spherical crater. Now, the military just loves antimatter, this magical cure-all for wars. A magical weapon that would make your enemies disappear has been the dreams of every militaristic nation. Now we have that capability, but it is still much too cost effective to create.