Antimatter is exactly the same as normal matter, but with its subatomic particles' quantum properties reversed, such as having the opposite spin and electrical charge. Antimatter happens to be rare in our universe, though trace amounts of antimatter are believed to be produced by powerful phenomena such as relativistic jets powered by black holes and pulsars. However, it is believed that in the Big Bang equal quantities of matter and antimatter were created.

When particles and anti-particles come into contact, both are annihilated with a tremendous release of high-energy photons (gamma rays) or other particle–antiparticle pairs. Because of this, and its ablity to interact with, modify, or destroy normal matter, anti-matter has been used in Star Trek episodes for everything from a fuel source and component of weapons of mass destruction,  to scanning technology and medical uses.  Anti-matter is certainly the primary power source of a starship's warp drive in Star Trek.  The warheads of a starship's photon torpedoes also contain antimatter - upon detonation the torpedo creates a matter-antimatter explosion.  

Chandra image of matter and antimatter propelled
to nearly the speed of light by the Crab pulsar

Because anti-matter particles are so extremely rare, real science has not yet been able to tap its huge potential as an alternative energy source. Generating a single antiproton, for example, is immensely difficult and requires particle accelerators and vast amounts of energy—millions of times more than is released after it is annihilated with ordinary matter due to inefficiencies in the process.  The current antimatter production rate is between 1 and 10 nanograms per year, and this is expected to increase to between 3 and 30 nanograms per year by 2015 or 2020 with new superconducting linear accelerator facilities at the European Organization for Nuclear Research (CERN) laboratories and Fermi National Accelerator Laboratory (Fermilab).

However, antimatter is currently harnessed in medicine for a few limited applications. A diagnostic technique called Positron Emission Tomography uses positrons, antiparticles of electrons, for scanning tis­sue. In addition, a form of radiation therapy uses pions, a hybrid form of matter and antimatter subatomic particles called quarks.

Acceleration and antiproton production at CERN.