Short answers, pulled from the story.
Unified field theory is a type of field theory that aims to describe all fundamental forces of nature and all elementary particles within a single mathematical structure. The term was coined by Albert Einstein, who pursued the idea from the 1920s onward. It remains an open problem in theoretical physics after more than a century of research.
Albert Einstein coined the term unified field theory. He began his attempts to unify general relativity with electromagnetism in the 1920s and continued the search for the rest of his life without producing a successful classical unified field theory.
The first successful unified field theory was James Clerk Maxwell's dynamical theory of the electromagnetic field, published in 1864. It unified electricity and magnetism, which had previously been treated as separate phenomena following observations by Hans Christian Orsted in 1820 and Michael Faraday in 1831.
Sheldon Glashow, Abdus Salam, and Steven Weinberg were awarded the Nobel Prize in Physics in 1979 for their work developing the electroweak theory. Carlo Rubbia and Simon van der Meer received the Nobel Prize in 1984 after Rubbia's team at CERN directly produced the Z and W bosons in 1983.
A Grand Unified Theory attempts to unify the strong, weak, and electromagnetic forces within quantum field theory but does not include gravity. A theory of everything aims to provide a complete description of all events in nature, including gravity, which requires reconciling general relativity with quantum mechanics.
The central obstacle is that general relativity and quantum mechanics are mathematically incompatible. Attempting to combine the graviton with the strong and electroweak interactions produces a theory that is not renormalizable, meaning its calculations generate unresolvable infinities. No widely accepted consistent theory bridging the two frameworks has been formulated.