A TACAN is a military only navagational aid that provides azimuth (radial) and range information (called DME or distance measuring equipment.) It is a non-precision aid, where an ILS provides precision azimuth (course), elevation or glidepath, and sometimes range (DME).
An ILS system can be certified to provide precision landing guidance in near zero-zero weather.
A TACAN can also be used in air-to-air mode, like when fighter aircraft need to locate a refueling tanker. The tanker can be a TACAN station for them to home in on.
You can equate a TACAN to the civilian VOR navaid. In fact, many TACAN and VOR stations are colocated, and are called VORTACs in that case. The benefit of a VORTAC to a civilian pilot is that they can use the DME feature of the TACAN to give them their distance from the station.
I hope I didn't confuse the subject too much.
73, Greg