An antiparticle is the inverse form of a particle. Each particle is predicted to have an antiparticle sibling, which is identical to the particle save for the fact that all quantum numbers are reversed. The mass remains the same.
An antiparticle can, in some rare cases, be identical to its particle. In this case it is said "the particle is its own antiparticle."
For example, the electron has an electric charge number of 1 and an electronic lepton number of 1. The positron, its antiparticle, has an electric charge of –1 and an electronic lepton number of –1. The mass of the electron and positron is the same. Meanwhile, the Photon is its own antiparticle.
When a particle and its antiparticle collide, they annihilate: this is a very energetic process that leads to the disappearance of the pair and the creation of high-energy photons.