QCD is quantum theory of fields for strong interactions.As the name suggest, it is written in the Framework of QFT. Its degree of freedom are Quarks and Gluons at very high energy(momenta)/small scales/temperature/densities, or hadrons at low energy(low momenta/large scales)/temperature/density.

QCD explains/calculates/predicts the following observables (which are in general true for all QFTs):

1. Scattering cross section for a given process.
2. Decay rates and decay constants.
3. Bound states and the properties of bound states.

Before I start explaining all these observables, let me state the properties of a particle and define what a particle is ?
The whole visible universe is made up of matter. Matter is made up of atoms (look at periodic table for more info about all kinds of atoms), all atoms are made up of nucleus (made up of varying number of neutrons and protons) and electrons around nucleus. The protons are made up of two up quark and one down quark, Neutron is made up of two down quark and one up quark. Here quarks and electrons are fundamental particles (not made up of any more fundamental constituents), where as protons, neutrons, nucleus, atoms, molecules can thus be called as composite particles(made from more than one fundamental particles).

Particles are theoretically point sized objects. These particles are defined as set of states which transform among themselves under Poincare transformations (Lorentz transformations + translation).

We would like to start the story with calculation of scattering cross-section of the
process:

\[ u+\overline{d}\to u+\overline{d} \]