The goal of high-energy nuclear physics is to understand the dynamics and properties of the various forms and phases of the strongly-interacting matter. Heavy-ion collision experiments are performed to deposit a large energy density in a very small volume of space and generate a form of extremely hot matter called the quark-gluon plasma (QGP). The behavior of the generated matter shows signatures of collectivity allowing phenomenological models based on statistical or fluid dynamical descriptions to be used successfully to analyze the outcomes the experiments. However, the very short lifetime and the extreme conditions of the QGP call for the construction of theoretical models based on the physics of nonequilibrium systems.Understanding the properties of QGP requires the study of collective excitations in the emergent many-body dynamics of the quarks and gluons as the fundamental objects in the theory of quantum chromodynamics. In this dissertation, the collective excitations of the nonequilibrium QGP are studied by calculating the quark and gluon self-energies within the hard loop effective theory. By extracting the solutions of the gluon dispersion relation in the complex plane, the presence of unstable modes in the momentum-anisotropic QGP is studied. The quark self-energy is also used to calculate the rate of photon emission from QGP in the subsequent studies performed as part of this dissertation. Electromagnetic probes (photons and dileptons) are considered among the best observables for extracting information about the early stages of evolution of the strongly interacting matter produced in heavy-ion collisions. In contrast to the hadrons, the emitted photons and leptons are not distorted by a strong coupling to the medium and they can escape the system with much larger mean free paths. Since the dilepton and photon emission rates from QGP are directly affected by the momentum distribution of the partonic degrees of freedom, their emission patterns can provide information about the nonequilibrium features of the system such as the momentum anisotropy of the distributions. In this dissertation, the effects of momentum anisotropy on the yields and elliptic flow coefficients of the dileptons and photons from QGP are investigated. The emission rates are calculated for distributions featuring ellipsoidal momentum anisotropies, and then the rates are convolved with the space-time evolution of the system using a relativistic hydrodynamical model. The effects of various parameters on the results are studied. In particular, the results are interpreted considering their connections to the extraction of early dynamics of QGP from future experimental data. Another important class of the probes of the nonequilibrium QGP is related to the dynamics of the heavy quarks in the system and their bound states. The properties of the heavy bound states, such as their decay rates, are affected by the hot and dense strongly interacting medium. Heavy quarks, due to their longer relaxation times, are also used to probe the transport properties of the QGP perturbed out of equilibrium. The potential for a bound state inside a hot medium may acquire an imaginary part. In this dissertation, the imaginary part of the heavy quark potential is calculated using classical-statistical simulations of the Yang-Mills theory on lattice. The connection of the results to the momentum diffusion of the heavy quarks is discussed. Lattice computation for the real-time dynamics of the non-Abelian theories allows for the nonperturbative analysis of the systems in and out of equilibrium.