Ethernet is the family of standards for wired local networks: cabling, connectors and the rules devices use to exchange frames. IP cameras, recorders, access controllers and workstations normally meet on Ethernet. Its practical limits are the 100-meter copper run, the switch port count and the bandwidth each link carries.
Ethernet is the plumbing under a modern security system. Cameras, recorders and controllers all assume it, and most installation problems trace back to it.
Frames, ports and speed
Devices exchange frames through a switch, which learns where each device sits and forwards accordingly. Links negotiate a speed, commonly 100 megabits or a gigabit. A camera needing more than its link allows will stutter or drop, and a failing switch is a common cause of intermittent faults.
Ethernet in a security network
Put cameras and recorders on their own switch and virtual local area network, separate from office traffic. Keep copper runs under 100 meters and use fiber beyond that. Label ports, document the addressing scheme and leave spare capacity for the cameras a site will add later.