An antiparticle has the same mass but opposite charge, baryon number, lepton number, and strangeness as it's corresponding particle.

Antiproton $\left(\overline{p}\right)$

Positron (e⁺ OR  β⁺)

antineutrino $\left(\overline{\nu}\right)$

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The mass of the positron is the same as the mass of the electron (9.11 x 10^-31 kg).

Particles have the same mass as their corresponding antiparticle.

During pair production a particle and it's corresponding antiparticle are produced from a single photon. 

When a particle meets it's antiparticle they annihilate. 

The particles cease to exist and their energy is split evenly between two photons.

Pair production

Annihilation

Rest mass energy is the energy of a particle due to it's mass alone.

Another way of phrasing it is the energy of a particle when it is stationary.