A proton is a elementary subatomic particle. It has one "unit" of positive charge (approximately 1.602 x 10-19 coulombs), and a mass of 1.67262 x 10-27 kg, about 1836 times heavier than an electron, and just slightly less than a neutron.

It, along with neutrons, makes up the nucleus of an atom. The number of protons in the nucleus determines the chemical properties of the atom - which element an atom is. A single proton forms the nucleus of Hydrogen (H+ is a proton without an electron).

A proton is considered a type of baryon, and is made up of 2 up quarks, and one down quark. It is also the lightest known baryon. It also has an antiparticle, an antiproton.

The existence of the nucleus of the atom was first theorized by Ernest Rutherford in 1911, in an attempt to explain his results during an experiment involving the scattering of alpha particles. In 1919, he discovered the existence of the proton, finding it as a product of the disintegration of the atomic nucleus. He gave it it's name, derived from the Greek word "protus", meaning first.

In 1965, Hofstadter and some collaborators used a linear accelerator to shoot high energy electrons at hydrogen, and using the deflection of electrons going by the proton to determine it's size - approximately 10-13 cm in diameter. This experiment also raised the first questions as to whether there was anything inside the proton (leading to quarks).

Some theories in physics have recently opened the possibility of proton decay - that is, that a proton may not be stable, and can eventually decay into lighter products. However, if this does occur, it takes an extremely long time to do so - experimental evidence has strongly suggested that protons have a minimum half-life of at least 1031 years - as no proton decay has ever been observed.

Encyclopedia.com, http://www.encyclopedia.com/articlesnew/10568.html
Proton: encyclopedia article from Wikipedia, http://www.wikipedia.com/wiki/Proton
Traveling to the Heart of Matter with HERA, Chapter 1, http://www.desy.de/f/hera/engl/chap1.html