Shares in companies based in mainland China that trade on either the Shanghai or Shenzhen stock exchanges. B-Shares are eligible for foreign investment provided the investment account is in the proper currency (Shanghai B-shares trade in U.S. dollars, while Shenzhen B-shares trade in Hong Kong dollars).
B-shares trade alongside A-shares in the Chinese companies on the mainland exchanges. Changes in government regulation have allowed Chinese citizens to invest in both A-shares and B-shares after previously limiting investment to only the A-shares.
B-shares are typically what a mutual fund or exchange-traded fund that invests in China will hold, along with H-shares from the Hong Kong Exchange and N-shares, which trade on the New York Stock Exchange.
As part of a long-term effort to open up China's economy, plans are in place for the two share types to be combined in the future to allow for more uniform investment policies; if and when this occurs it should encourage more outside investment in the world's most populous country.