Some processes and reactions in our universe naturally produce tons of energy. Nuclear fusion and fission are a good example. The process of splitting or combining atoms to form matter is a dangerous, energy filled process. Matter in general has a lot of energy contained within the bounds of its different particles.
Massive amounts of energy can also be produced when certain particles collide and interact. This type of reaction is partly how nuclear weapons work, which obviously contain scary amounts of energy, enough to annihilate hundreds of thousands of people in an instant. Another type of collision that produces massive amounts of energy is when matter and antimatter collide.
Before getting into specifics, what are matter and antimatter? Matter is everything around us. Everything you touch or see has matter in it. Matter is made of building blocks, specifically atoms. Atoms are made of electrons, protons, and neutrons. These three fundamental particles can further be divided into even smaller particles called quarks, gluons, and leptons. Whether there are smaller particles is unknown, though it seems likely that there are. The world simply does not have any instrument powerful enough to detect them yet.
Antimatter is an interesting phenomenon, one still not fully understood. In regular matter, each fundamental particle has an electric charge. The electron is negative, the proton is positive, and the neutron is negative. Antimatter particles simply have the opposite electric charge that their corresponding matter particles have. For a negatively charge electron, you have a positively charged positron. For a positively charged proton, you have a negatively charged anti-proton. With neutrons, it gets to be a little trickier. The anti-neutron also has no charge. The difference between the two lies in the quarks comprising the neutron. A neutron is made of up quarks and down quarks (these are different types of quarks, there are more than just two types). An anti-neutron is made of anti-up quarks and anti-down quarks. Another note about antimatter, the antimatter particles must have the exact same mass as their counterparts. Their electric charge must be exactly the same amount, just opposite.
When matter and antimatter collide, they annihilate each other. They cannot coexist in each others presence. They cancel each other out because they have the exact same mass and exactly opposite charges. When an electron collides with a positron, the resulting release of energy is massive. Two photons form and carry away the energy, which is about 0.5 million volts. This is an absolutely massive amount of energy and is released in a spectacular blast.
When compared to a nuclear fission bomb, a maximum of 0.1% of the plutonium core can become this explosive energy. A fusion bomb can achieve a maximum of about 0.5%. Antimatter and matter collisions have the potential to reach 100% conversion to energy. This ridiculous figure is because both the matter and antimatter are fully obliterated by their interaction, leaving no mass behind. It is all converted to energy. To get some hard numbers, if one kilogram of matter was hit by 1 kilogram of antimatter, the resulting explosion would have the force of about 43 million tons of TNT. This is equivalent to several thousand Hiroshima nuclear bombs exploding at once.
After the collision occurs, the two particles vanish into high-energy gamma rays. Some harmless neutrinos are also discharged. They do not really interact with anything and can even pass through whole planets without any effect.