BHP and BBL are the US ADR's for BHP Billiton Ltd, Australia [BHP.AX] and BHP Billiton Plc, United Kingdom [BLT.L].
These two companies are what is known as Dual Listed Companies. Instead of merging, they agree to combine all assets, employees, incomes, management and Directors, consolidate their books in US Dollars, and operate as if merged.
The US ADR's represent two of the underlying company's shares. Each has two votes and get twice the dividend of the underlying shares. So in effect there is no difference in the ADR's except their shareprice. No one knows why.