ADR is method of trading non-U.S. stocks on U.S. exchanges
Suppose, Indian Co. wants to raise money from America, by issuing shares in American stock exchange.
But then Indian co. will have to maintain accounts according to American standards.
To prevent this problem, Indian company gives its shares to American bank.
American bank gives that Indian company receipts (called ADR) in return of those shares. Then Indian Co. can trade those ADR receipts in American share market, to raise money.
Serve as same function like ADR, but on Global scale, it helps the countries from third world, to raise money from the stock exchanges in developed countries.
Normally 1 GDR = 10 Shares, but not always.
##As described by Mrunal