In the standard model for describing fundamental particles and interactions, every particle has an antiparticle. For example, the positron is the antiparticle of the electron. It has identical mass, but has a positive charge. If an electron encounters a positron, they annihilate with the transformation of their mass energies into two gamma rays.
Going beyond the basics, we can say that an antiparticle is related to the particle by charge conjugation. This includes more than just electric charge; it inverts all internal quantum numbers such as baryon number, lepton number, and strangeness.
Seeing that nature is in so many respects symmetric, a fundamental question is "Why is the universe made out of matter, with very little antimatter? We can create antimatter today in pair production, and in fact the processes of particle production produce equal numbers of particles and antiparticles. Something very early in the history of the universe tipped the balance toward matter to the virtual exclusion of antimatter. We see antimatter today only in pair production, radioactive decay and in the products of the high-energy particle accelerators.
The question is an open one. Current research which may shed some light on the question is that which explores the apparent violation of CP invariance in the decay of the neutral kaon. At the fundamental particle level, nature can tell the difference between left-handed and right-handed systems as indicated by parity violation with neutrinos (neutrinos are left-handed and antineutrinos are right-handed). While normal beta decay violates parity, it is invariant under CP. If the neutral kaon is confirmed to directly violate CP, it may provide an avenue for nature to show a preference for matter over antimatter in the early universe.
Das & Ferbel