VOR stands for Very High Frequency Omnidirectional Range. It is a radio transmitter capable of sending signals resembling the spokes of a bike wheel. Basically the system consists of two parts: the ground station and the receiver in the aircraft with indicator, either mechanical or on a nice multi function display.
The objective is to show the pilot on which spoke, of the wheel, he/she is flying and in which direction. It thus provides course guidance, automatic wind correction and magnetic headings.
Eurocontrol published a document containing VOR/DME/ADF radio navigation aids.
Eurocontrol Guidance Notes
Excerpt from the document:
The three most common types of radio navigation aid are: VOR indicates the magnetic bearing from the station to the aircraft (the Radial). The original instrument, the Omni-Bearing Selector (OBS) indicates the aircraft's position relative to the radial the pilot has selected, and requires careful interpretation.
ADF (Automatic Direction Finding), indicates relative bearing to a radio beacon, called an NDB (Non-directional Beacon). The original RelativeBearing Indicator (RBI) only shows that relative bearing, and can still be found.
DME (Distance Measuring Equipment) indicates the distance in nautical miles from the beacon to the aircraft. The display is usually digital.
To continue reading see the pdf below: