Very Long Baseline Interferometry (VLBI) is a geometric technique: it measures the time difference between the arrival at two Earth-based antennas of a radio wavefront emitted by a distant quasar. VLBI precisely locates points on Earth by comparing radio signals from a quasar radio source located in deep space received at different times at different Earth ground stations. This method gives relative positions of the Earth stations. VLBI data can be used to calculate information on Earth rotation, length of day, polar motion, baseline and velocities of stations. Products generated by VLBI contribute to research in many areas, including solid Earth, tides, studies of the vertical, fundamental astronomy, and VLBI technique improvement.