  The observation of quark and gluon jets has played a crucial role in
establishing Quantum Chromodynamics [QCD] as the theory of the strong
interactions within the Standard Model of particle physics. The jets, narrowly
collimated bundles of hadrons, reflect configurations of quarks and gluons at
short distances. Thus, by analysing energy and angular distributions of the
jets experimentally, the properties of the basic constituents of matter and the
strong forces acting between them can be explored. In this review, which is
primarily a description of the discovery of the quark and gluon jets and the
impact of their observation on Quantum Chromodynamics, we elaborate, in
particular, the role of the gluons as the carriers of the strong force.
Focusing on these basic points, jets in $e^+ e^-$ collisions will be in the
foreground of the discussion and we will concentrate on the theory that was
contemporary with the relevant experiments at the electron-positron colliders.
In addition we will delineate the role of jets as tools for exploring other
particle aspects in $ep$ and $pp/p\bar{p}$ collisions - quark and gluon
densities in protons, measurements of the QCD coupling, fundamental 2-2
quark/gluon scattering processes, but also the impact of jet decays of top
quarks, and $W^\pm, Z$ bosons on the electroweak sector. The presentation to a
large extent is formulated in a non-technical language with the intent to
recall the significant steps historically and convey the significance of this
field also to communities beyond high energy physics.
