A circuit is an end-to-end pathway for the transmission of data. Circuits can be logical or physical.
Logical circuits allow for end-to-end communication without specifying a particular physical pathway. Different portions of the total message may traverse different physical pathways. As long as some physical pathway is open for each portion of the message, different pieces of the message can be reassembled on the receiving end. The communication is thus abstracted from the details of the physical links: physical links can be changed or substituted as needed. This type of circuit that abstracts from physical links is called logical or virtual.
Physical circuits provide end-to-end communication through some medium or carrier. Examples of such media include electronic, fiber optic, and radio frequency (RF) communications.