Baryons are a category of subatomic particles in the nucleus of the atom. Protons and neutrons are examples of baryons. Every atom has at least one baryon in its nucleus—at least a single proton. The element with a single proton in its nucleus is the common form of the element hydrogen. Other elements have atoms with more than one proton and more than one neutron.
Baryons are not elementary particles; they are composite, that is, they have components. All baryons are composed of three quarks held together by the strong force. If a particle is composed of three quarks, it’s a baryon. As they’re composite, baryons are not elementary (fundamental) particles.
Spin. Baryons spin as an intrinsic property. That is, they are “born” spinning and continue spinning as long as they exist.* Spin is a property of subatomic particles that follows the laws of quantum physics, rather than the laws of the physics we are used to in the everyday world (classical physics). Baryons have spin one-half. “Spin one-half” means that, in some sense (that probably no one has yet figured out how to visualize), they twirl around and return to their original position after making only half a revolution. If we were to speak of objects in our everyday world as having spin, we would say that they have “spin one.” After one revolution, they’re back in their original position. Of course, in our everyday world, objects aren’t born spinning. They don’t have inherent spin; they get spin from a force operating on them.
Hadrons. Baryons are one of the two types of hadrons, the other type being mesons. Mesons are not made up of three quarks; they are subatomic particles made up of a quark and an anti-quark.
*Subatomic particles can come into existence (get “born”) at any time. They may have been born when particles started forming about 300,000 years after the Big Bang. Or they may be born as the result of an interaction of other particles or as a result of the decay of another particle. Similarly, they may blink out of existence due to “unfortunate” interactions. Throughout these interactions, energy is conserved.