We review lattice QCD results on the order of the phase transition, me
son correlators, and thermodynamic functions for temperatures around t
he deconfinement phase transition temperature T-c. Non-perturbative ph
enomena are found to dominate the behaviour of the quark-gluon plasma
up to temperatures of about 2T(c). Thermodynamic properties of the pur
e SU(3) gauge theory can be explained by the existence of massive gluo
nic quasi-particles or bound states of gluons (''gluon clusters'').