An ADR is a dollar-denominated negotiable certificate that represents ownership of shares in a non-U.S. company. The structure of an ADR includes a ratio, which correlates the amount of underlying shares to the receipt. An ADR can be canceled for its underlying shares at anytime.
ADS
An ADS is the instrument that is actually traded. It represents the underlying share, which trades in the issuer's home market. Although the terms ADR and ADS are used interchangeably (including in this Reference Guide), the difference is that an ADR is the certificate while ADSs are the shares.