This is a dated thread, but here's how I would define them:
A product line is a progressive build. Each person does one operation in the build process and moves it on until it is complete.
A work cell is a self contained unit where a complete finished good is made, often by individual operators. The cell contains all the raw materials needed to build, KanBan carts supplied to the operators on a pull basis and final inspection or operator self-inspection. A cell is usually a lean operation, taking up less floor space and requiring less operator down-time.