Load wires are the two fat wires in the electric light cable. Line wires are thinner and provide a higher current for appliances that require more power such as a toaster.
Line wires generally carry more voltage per wire than load wire, because they deliver higher currents at a lower cost-per-ampere (wires can be transmitted over longer distances). Switching appliances on or off usually requires one line, but sometimes two line are needed. Multiple load wires typically carry less current and use less energy without any special switching requirements, but may need individual switches that correspond with their applications (for example, there might be an outlet for lighting fixtures in one room of the house and an outlet for electric heating units in another).
