Many types of hadrons were discovered in the 50's and 60's. The idea of the quarks was first proposed to explain the many observed hadrons. Its basic structure is two up quarks and one down quark .
Neutrons are also baryons.
Although neutrons are not stable in isolation, they can be stable inside certain nuclei. A neutron's basic structure is two down quarks and one up quarks. More massive baryons may be made from any set of three quarks. Baryons containing more massive quarks are all unstable because these quarks decay via weak interactions. There are also more massive baryons that have the same quark content as a proton or a neutron but have additional angular momentum.