A relay is an artificial bid asking the partner to give more information. Typically, this involves making the next available bid. Conventions that use relays are
Additionally, a number of systems make extensive use of relays. These are called relay systems, which are classified as artificial.
According to ACBL, a system built completely on relay bids (one player tells nothing about his hand in bids, only relay) is not permitted in ACBL General Convention chart. It's allowed to play in mid-chart for game forcing sequences only and allowed without restriction in super chart.