Nails and screws are fasteners that hold wood or other materials together. Both nails and screws are driven into the wood and then held by friction and/or by a thread or threads in the hole.
The main difference between nails and screws is that nails are made from much softer metal than screws and so they leave a nail hole when they are removed, while screws do not. Both nails and screws have a head on one end and a shank on the other, and so both can be driven with a nail or screwdriver.
Both nails and screws come in different sizes and lengths, and so both can be used to fasten materials of different thicknesses. Both nails and screws can be driven by a hammer or a power driver, and so both can be used to fasten materials that are hard to reach.