The internal structure of a proton, according to our current understanding, is described by the theory of quantum chromodynamics (QCD). According to QCD, a proton consists of fundamental particles called quarks, specifically two "up" quarks and one "down" quark, bound together by the strong nuclear force.
Quarks are elementary particles and are considered to be the building blocks of matter. They have fractional electric charges and come in different "flavors" or types. The up quark has a charge of +2/3, while the down quark has a charge of -1/3.
However, it is important to note that the concept of "inside" the proton can be misleading. Protons, along with neutrons, are subatomic particles known as hadrons, and they are composite particles composed of quarks held together by the strong force. The strong force is mediated by particles called gluons, which bind the quarks together within the proton. This interaction between quarks and gluons is complex and nontrivial, making it difficult to describe precisely how they are arranged within the proton.
In summary, a proton is composed of three quarks (two up quarks and one down quark) held together by gluons via the strong nuclear force.