When a company based overseas wants to sell its shares in the US markets, it can offer them through a US bank, which is known as the depositary.
The depositary bank holds the issuing company's shares, known as American depositary shares (ADSs), and offers them to investors as certificates known as American depositary receipts (ADRs). Each ADR represents a specific number of ADSs.
ADRs are quoted in US dollars and trade on US markets just like ordinary shares. While hundreds are listed on the major exchanges, the majority are traded over the counter, usually because they're too small to meet exchange listing requirements.