The Standard Model is a theory of fundamental particles and how they interact. It does not, as yet, include gravity, which I address in the next section, Gravity, as well as in the section on Quantum Gravity. Developed initially in the 1970's, it is a well established theory applicable over a wide range of conditions, and experiments have shown it to be incredibly accurate. The following sections describe the particles and forces included in the Standard Model. The model predicted seven of these particles prior to their discovery. All of these particles have been identified experimentally except the Higgs Boson. Below, there is a video that summarizes the Standard Model.
The Standard Model is a point particles theory; that is, it envisages particles as dimensionless dots. This is fundamentally at odds with much recent work in the field; particularly Superstring and M Theories. To date, all attempts to merge any point particle theory with general relativity has resulted in failure as the results become plagued with infinities.
A quick note about atoms. The nucleus of an atom contains a number of protons that define the element; hydrogen, gold, lead etc. The nucleus also contains a number of neutrons, similar to the number of protons. The number of neutrons defines the particular isotope of the element. Finally, there are the same number of electrons around the nucleus as there are protons, in the neutral state of an atom. If electrons are lost or gained, the atom becomes an ion, or is said to be ionized. The scale of an atom is interesting. The thickness of a human hair is equivalent to about one million atoms of carbon. The nucleus, however, comprises just one ten-thousandth of the diameter of an atom. So, if we magnify the nucleus of a hydrogen atom, a single proton, to a diameter of one centimeter, the electron would be about half a km away (if the nucleus were an inch in diameter, the electron would be just over ¾ mile away).