In the early 1800, Dalton proposed six basic rules for Chemistry. One of his postulates was that atoms are the smallest pieces of matter and can not be broken into anything smaller. Subsequent developments in our understanding of matter have shown us that this particular postulate of Dalton's has to be modified in that matter can be broken down into pieces smaller than atoms.
Between the late 1800s and mid 1900s the structure of the atom was developed with the discovery of electrons, protons, and neutrons along with the understanding that protons and neutrons are sequestered within the small, dense nucleus and the electrons in the electron cloud outside the nucleus.
In the 1960s it became clear that not only can the atom be broken down into its 3 basic components, the proton and neutron could be broken down even further. It was already understood that a neutron can split into a proton and electron (so called "beta" decay), but it has now been learned that protons themselves can be broken into pieces.
The theory has been established that a proton can be broken down into three smaller pieces called quarks. Further, these quarks have fractional charges which must add up to +1 to account for the +1 charge of the proton.
Applying a property of nature known as symmetry, the theory has expanded to include six quarks which are to date the smallest pieces of matter that can be combined to produce all types of matter.
The six quarks are given common names to help remember the names and to reflect their symmetric nature: top, bottom, up, down, strange, and charm. The proton is formed by the combination of two up quarks (each with a charge of +2/3) and one down quark (charge -1/3)