VU Meters 

In 1939 Bell Labs, NBC, and CBS together developed a meter to display the volume of an audio signal. Original it was called the SVI (Standard Volume Indicator) meter, but later became known as VU an abbreviation for volume units. It works by averaging the volume over a short period of time (approximately 300 ms). These meters are found on analog tape recorders, mixing consoles, and outboard gear.
Because VU meters show an average volume level, steady-state sounds will display the volume fairly accurately. However, audio signals that have intermittent silence (like dialog or drums), will have a VU reading lower than the actual amount of peak energy in the recording.
By contrast, the dBFS (db Full Scale) meters used to display volume on digital audio equipment have more in common with the PPM (peak power meters) that were in use even before VU meters. Peak power meters will show transient spikes that might be missed on VUs.
Generally, with pro gear 0 VU is +4 dBu and with semi-pro (consumer) gear, 0 VU is -10 dBu.
And that's another big difference between VU and dbFS meters. VU meters are adjustable. Since they build in headroom, they can be adjusted to whatever operating level is being used. Whereas dBFS meters are absolute. 0 dBFS is the same on every system, everywhere, because there is no additional headroom.
VU meters are much less relevant with digital gear than they were with analog equipment, leaving some people thinking that VU actually stands for "virtually useless".