Antimatter is a form of matter composed of antiparticles, which are the counterparts to the particles that make up ordinary matter. Antiparticles have the same mass as their corresponding particles but opposite electric charge and other quantum numbers.
Every particle in the standard model of particle physics has its antiparticle counterpart. For example, the antiparticle of an electron (a negatively charged particle) is called a positron (a positively charged particle with the same mass as an electron). Similarly, the antiparticle of a proton (with a positive charge) is called an antiproton (with a negative charge and the same mass as a proton).
The key difference between particles and antiparticles lies in their electric charge. In addition to the opposite charge, other properties, such as the magnetic moment, may also have opposite signs in antiparticles compared to their corresponding particles.
When a particle and its antiparticle meet, they can annihilate each other, converting their mass into energy. This process releases a tremendous amount of energy and is an important concept in both theoretical and experimental physics.
In summary, antimatter consists of antiparticles, which have the same mass as their corresponding particles but opposite electric charge and other quantum numbers.