Short answers, pulled from the story.
The Standard Model of particle physics is the theory describing three of the four known fundamental forces, the electromagnetic, weak, and strong interactions, while excluding gravity. It classifies all known elementary particles, including twelve fermions and four kinds of gauge bosons.
The Standard Model was developed in stages throughout the latter half of the 20th century through the work of many scientists worldwide. Its current formulation was finalized in the mid-1970s upon experimental confirmation that quarks exist.
The Standard Model includes twelve fermions, split into six quarks and six leptons, plus four kinds of spin-1 gauge bosons. The quarks are up, down, charm, strange, top, and bottom, and the leptons are the electron, muon, tau, and their three neutrinos. It also includes the spin-0 Higgs boson.
The Standard Model does not describe gravity because quantum mechanics and Einstein's general relativity are incompatible. The graviton has been proposed as gravity's mediating particle but has never been observed.
On the 4th of July 2012, the ATLAS and CMS experiments at the Large Hadron Collider independently reported a new particle with a mass of about 125 GeV, consistent with the Higgs boson. On the 13th of March 2013, it was confirmed to be the searched-for Higgs boson.
The Standard Model is considered incomplete because it does not explain gravity, dark matter, dark energy, or the predominance of matter over antimatter. It also requires 19 arbitrary numerical constants and did not originally allow neutrino mass, which experiments have since shown to exist.