Radio Frequency - Literally means any of the range of frequencies used to transmit radio signals through the air. This is often used as a broad synonym for "radio", "radio waves", "radio transmissions" and similar things. It is used for example in the term RFID, which stands for Radio Frequency Identification.

RF is the transmission medium used by WLANs and (as mentioned) RFID tags. RFID tags generally use the energy from radio waves radiated toward them to reflect back information contained in the tag.