ADF stands for Automatic Direction Finder while VOR stands for Very High-Frequency Omni Directional Radio Range. VOR is known to be the radio navigation system that is being used by aircraft that are also operating with the use of the VHF. The ADF is very helpful especially when you are trying to find the current bearing of the aircraft. Both VHF and ADF are considered to be an old air navigation system.
Between the two, VOR is considered to be the more advanced version. This has been used since the 1960s and is still being used by a lot of people right now. These navigation systems are still trusted by a lot of people from all over the world.