An air dryer is a device that removes moisture from compressed air to prevent damage to air tools and machinery. There are two main types of air dryers: desiccant air dryers and refrigerated air dryers. Desiccant air dryers use a material that absorbs moisture from the air stream, while refrigerated air dryers use a cooling process to condense and remove moisture. Both types of air dryers can effectively prevent moisture buildup in an air compressor system.