At their most basic, wire harnesses are a simple exterior sheath that covers some wires or cables. It’s usually made of thermoplastic or thermoset material which helps protect the wires from their environment. For the most part, wire harnesses bundle up cables and wires that are already protected by their own exterior sheaths. That means you could open up a wire harness and see multiple cables with their own coverings, but they’re all in a single wire harness.
Wire harnesses are a low-cost electrical solution that’s best for keeping electrical systems organized. By bundling wires and cables together, engineers and electrical contractors can keep their wiring systems organized. Since wire harnesses are often a low cost organizational tool, they aren’t generally able to protect the wires inside from constant friction, temperature swings, or other factors that could cause damage to the wires inside the harness.