What is the difference between a VOR and a VORTAC?
A VOR ground station (VHF Omni-directional Range) provides azimuth information to aircraft equipped with a VOR receiver.

A TACAN facility (Tactical Air Navigation) provides azimuth and distance information to the facility in slant range (known generally as DME) using the UHF (Ultra High Frequency) band to TACAN equipped aircraft (normally military aircraft).   

A VORTAC (VHF Omni-directional Range/Tactical Air Navigation) combines a VOR ground station and a TACAN ground station into a single facility.  

If a VOR equipped aircraft is also equipped with a separate DME unit it will receive distance information between the TACAN ground station (at the single combined facility) and the aircraft.

More technical and expanded information can be reviewed beginning with section 1-1-3 in the Aeronautical Information Manual (AIM)