Before revision: These fittings act similar to the way regular compression fittings work, but use a resilient O-ring for sealing, and a grip ring to hold the (possibly grooved) tube in place.
After revision: These fittings act similar to the way regular compression fittings work, but use a resilient O-ring for sealing, and a grip ring to hold the (normally EPDM)(normally stainless steel) tube in place.