This symbol represents a VORTAC (VHF
Omni-directional Range & Tactical Air Navigation) station. The differences are
VOR - VHF Omni-directional Range. This radio transmits signals that provide magnetic bearing information from the location of the transmitter. According to the signal you receive, you can determine what magnetic direction you are located in reference to the transmitter. In other words, if you were flying on the NORTH side of the transmitter, you would receive signals indicating that you were located on a NORTHERN magnetic bearing from the VOR transmitter such as 350 or 010 degrees, etc.. These signals radiating from the station are called RADIALS and each VOR has 360 radials aligned with magnetic NORTH as reference.
VOR/DME - VHF Omni-directional Range / Distance Measuring Equipment. This station provides VOR signals as well as Distance information. Our aircraft MUST have the DME receiving equipment to use the DME signal, most do. Not only can we determine what magnetic direction we are located FROM the VOR transmitter, we can also receive signals indicating our Distance in Nautical Miles. By having this distance information, we can calculate accurate ground speed calculations that can help determine our Estimated times, fuel consumption, winds, etc..
TACAN - Tactical Air Navigation This is the US Military's version of VOR/DME. Using a UHF transmitter and a special TACAN receiver, this system operates similarly to the VOR/DME. TACAN includes the DME information along with its magnetic bearing signals. TACAN radios can receive ONLY TACAN stations or DME information from a VOR/DME station. No VOR information can be received on a TACAN radio due to the use of UHF instead of VHF. So, if a Military pilot needs to land at an airport that offers only a VOR/DME Naviad, he ,can use his TACAN radio to receive the DME information, and must use an additional VHF radio to tune in the VOR signal. His TACAN radio can't provide the Magnetic Bearing information provided by the VOR station.
NDB - Non-directional Beacon. A simpler navigation system that is becoming outdated. This system uses a transmitter that radiates a signal on a particular frequency in a uni-directional pattern (like a light-bulb does in a lamp). A needle on the ADF (Attitude Direction Finder) Indicator in the aircraft will point to the direction that has the 'brightest' reception. This provides only Relative Bearing information, for instance you might simply know that the station is 90 degrees to your left (which may actually be 342 degrees magnetic).
DME - Distance Measuring Equipment. A radio system that provides distances between the aircraft and station in Nautical Miles. More elaborate DME equipment can take this information and provide ground speed to/from the station also.