Summing with multiple conditions
=SUMIFS(sum_range, criteria_range1, criterion1, [criteria_range2, criterion2, ...])
SUMIFS adds the cells in sum_range for rows where every condition is met.
Heads-up: the argument order is different from SUMIF. In SUMIFS the sum range comes first. This trips up nearly everyone at least once.
=SUMIF(criteria_range, criterion, sum_range) =SUMIFS(sum_range, criteria_range, criterion, ...)
The sum range can also be a criteria range, e.g. "paid invoices of $500 or more".