The Weak force is one of the four fundamental forces of nature. It is responsible for beta decay, the most ubiquitous form of radioactive decay. It is transmitted by three bosons, the uncharged Z boson and two W bosons, one having positive charge and the other negative.
The weak and electromagnetic forces have been unified into one force, called the "electroweak" force. This was done around 1973 by Steven Weinberg, Abdus Salam, and Sheldon Glashow, for which they won the 1979 Nobel Prize. This unification was vastly more complex than the unification of the electric and magnetic forces, worked out by Michael Faraday and James Clerk Maxwell a century earlier.