A “field" in physics is a region of space in which there is some physical influence on objects. Examples include gravitational and magnetic fields. A field theory therefore describes how these fields behave, and how objects interact with the fields they are in. One of the founders of quantum mechanics, Paul Dirac, published several papers in the late 1920s showing how quantum theory could be combined with James Clerk Maxwell's field theory of electromagnetism, as well as with Einstein's special theory of relativity. What he produced was the first "quantized" field theory, which described how electrons and photons, the particles of light, interact with each other.After a promising start, quantum field theory had a rough time during the 1930s and 1940s when it was plagued by mathematical difficulties. These were finally sorted out when, in 1949, several physicists, including the great Richard Feynman, produced quantum electrodynamics, or QED, for short. Later, this theory was used to combine the electromagnetic force with another of the four forces of nature-the weak nuclear force. This development became known as the electroweak theory. A separate quantum field theory, known as quantum chromodynamics, has also been developed to describe the strong nuclear force. Only the fourth and final force of nature-gravity-has, thus far, resisted attempts to be quantized.
by Jim Al-Khalili, 30 Second Theory