Why can salt be used as a weed killer (Osmosis)?

Salt kills plants because the cells of plants contain water and other dissolved things. When surrounded by water that contains a larger amount of salt there is a general rule that water will try to move between the inside and the outside of the cell wall to make the concentration of dissolved things equal. This movement of water to equalize concentrations of dissolved things is called osmosis. So a high concentration of salt outside the cell wall will suck the water out of the cell and eventually cause the cell to collapse (which kills the plant).