vlookup will do this.
it would look something like =vlookup(A:A,Sheet1!A:A,B:B,False)
Sorry, I typed that wrong. It should be =vlookup(A:A,Sheet1!A:B,2,False)
The function syntax is vlookup(lookup_value,table_array,col_index_num,[range_lookup]
lookup_value = is the value you want too look up.
table_array = the table or range that you want too look up the information on (note that the 1st column in that table/array HAS to be the column that you are looking in for your lookup_value.)
col_index_num = which column from the array that excel will return when it finds the value in the first column of the array.
[range_lookup] = if true, it will find an approximate match from your array. if false, it will find an exact match from your array.
So in the equation "=vlookup(A:A,Sheet1!A:B,2,False)" You are telling excel to look for whatever is in column A of the same row that the active cell is in. You are telling it to look on sheet1 in column A. You are telling it to return the second column from A:B and that you want an exact match.
Hope that's what you were looking for.