Sum is pronounced as "soom". Quod is pronounced as "kwod", with a short 'o' (note to Americans: not the common American short 'o' that verges on 'ah'; more like the 'au' in "taught").
Sum is a general operand. Sum if requires certain conditions to be met. For example - you could have a list of daily sales for a year. Sum will total all the figures to give an annual total. Sum if...