  Jets are observed in young stellar objects, X-ray sources, active galactic
nuclei (AGN). The mechanisms of jet formation may be divided in regular, acting
continuously for a long time, and explosive ones. Continuous mechanisms are
related with electrodynamics and radiation pressure acceleration,
hydrodynamical acceleration in the nozzle inside a thick disk, acceleration by
relativistic beam of particles. Explosive jet formation is connected with
supernovae, gamma ray bursts and explosive events in galactic nuclei.
Mechanisms of jet collimation may be connected with magnetic confinement, or a
pressure of external gas. Explosive formation of jets in the laboratory is
modeled in the experiments with powerful laser beam, and plasma focus.
