Hi
A reed switch is normaly operated by proximity of a permanent magnet. As the magnet is brought up to the reed it will change its state depending on whether it is normaly open or closed. Often used in door alarm systems.
A reed relay is a reed switch with a coil wound normally round the reed when the the coil is energised the reed will again change its state.
Reed units can be normaly open, normally closed, or single pole change over. One advantage of a reed is the speed of operation, several of the cheaper aneometers use a magnet on the rotor and reed mounted in the base, as the thing turns the reed operates and the pulses are counted by the electronics.
Like normal relays they come in various sizes and current ratings.
Brian