-Load wires are the low voltage line (typically 12VDC) wiring running to a load.
-Line wires are parallel circuits providing a higher voltage power to other loads. For example, the output of a generator is typically 110VAC which would be carried on two 240V line wires and one ground wire.
A building is usually wired with both loads and lines in order to have the highest reliability and safety levels possible while still allowing flexibility during installation.