Introduced to the financial markets in 1927, an American Depository Receipt (ADR) is a stock that trades in the United States but represents a specified number of shares in a foreign corporation. ADRs are bought and sold on American markets just like regular stocks, and are issued/sponsored in the U.S. by a bank or brokerage.
Primarily the difficulties associated with trading at different prices and currency values. For this reason, U.S. banks simply purchase a bulk lot of shares from the company, bundle the shares into groups, and reissues them on either the NYSE, AMEX, or Nasdaq.
The depository bank sets the ratio of U.S. ADRs per home country share. This ratio can be anything less than or greater than 1. The reason they do this is because they wish to price the ADR high enough as to show substantial value, yet low enough, so that the individual investors can purchase these shares.
Most investors try to avoid investing in penny stocks, and many would shy away from a company trading for 50 Russian Roubles per share, which equates to $1.50 US per share. As a result, the majority of ADRs range between $10 and $100 per share. If, in the home country, the shares were worth considerably less, then each ADR would represent several real shares.