It consists of physical media like copper wires, optical fiber. It modulates the data link layer data with necessary encoding like NRZ/Manchester and transmits data onto the media. It may use amplitude or frequency modulation technique. It uses several code groups called symbol like 4B/5B so that number of 0s and 1s remains balanced. Otherwise there might be power loss. The bandwidth depends on media, the type of connectors used etc.
There are several forms of media like:
10 Base T: 10Mbps using Manchester encoding. Cu media
100 Base TX: 100Mbps Cu media.
1000 Base TX: 1Gbps Cu media.
1000 Base SX/LX: 1Gbps Multi mode Optical Fiber.
1000 Base ZX: 1Gbps Single Mode Optical Fiber.
The copper cables are
UTP: Unshielded, twisted so that to reduce interference
STP: Shielded not used commonly due to high cost