Microsoft Excel: Functions - SUMIFS and COUNTIFS
These two functions are great for summarising data, producing running totals that you can then refer to with no further effort.
SUMIFS totals number under certain circumstances. For example, you might have a team of sales people selling different flavours of ice-cream. Using SUMIFS you could see at a glance the value of sales by person and flavour.
COUNTIFS is similar but it would count the number of orders of each flavour by sales person.
To use these effectively you really need to understand Absolute References (using a $ in a formula). Include that module if you aren't sure.