The Standard Model describes the universe in terms of six Quarks, six Leptons and interacting force carriers called Bosons. The Photon, Gluon and the Z and W Bosons mediate the Electromagnetic, Strong and Weak forces respectively. The theory does not contain a Gauge description of Gravity but can explain the acquisition of mass through the Higgs Mechanism. The model is sub-divided into three generations of particles and every particle listed has a corresponding anti-particle. The three generations of Quarks include the Up and Down Quarks, Strange and Charm Quarks, Bottom and Top Quarks. The generations are ordered according to masses and similar properties between the Quarks.
The Top Quark, herein referred to as the Top, is the heaviest Quark in the Standard Model with all other Quark masses being two orders of manitude smaller. The Top is highly unstable and prompt in its decay making isolation impossible. Instead particle physicists must study the particles the Top decays into in order to infer its existance and study its characteristics.
The Standard Model predicts with almosts 100% accuracy the decay of a Top into a W Boson and Bottom Quark. The Bottom Quark is also unstable but lives long enough to fragment through QCD interactions. The spray of light particles then produced is referred to as a Jet.
The W Boson is a massive particle and so undergoes further decay, either to a Lepton and same generation Neutrino or to a light Quark Anti-Quark pair. Trivially we can thus construct the two decay routes for the Top; the Hadronic decay to a pair of light Quarks and a single heavy Quark; the Leptonic decay to a Lepton and Neutrino plus a single heavy Quark.
Top Quark studies are made at particle colliders, such as at the Large Hadron Collider where eighty million Top Quark pairs will be produced annually at design luminosity. For Top Quark pairs we can therefore have three decay routes; Hadronic resulting in four light Quarks and two heavy Quarks; Lepton plus Jets resulting in a single Lepton and Neutrino, two light Quarks and two heavy Quarks; Di-Lepton resulting in two Leptons, two Neutrinos and two heavy Quarks. The Hadronic route accounts for 46% of all Top pair decays and on first glance appears a good choice for Top studies at colliders. However QCD interactions can emulate the signals produced in this scenario so the single Lepton plus Jets channel is more feasible for intense studies.