Cummulative sum against Condition
 

I would like to calculate the sum of column B if the cells against that on
column A satisfy my condition. In column A i have about 8 to 10 types of
entries with some quantites against each entry in column B. The sum shall be
calulated in a separate sheet say 'sheet 2'. Every time i enter a value in
column A & B in 'Sheet 1'. The corresponding sum shall be updated against
that type of entry in 'sheet 2'. Vlookup only gives the vakue against one
type but does not consider if the same type appear later in that column.
Please suggest a suitable formulae for this calulation.