In math terms, sum and total probably mean the same thing. But:
"The sum of 4 and 2 is 6." sounds much more natural than
"the total of 4 and 2 is 6."
Total seems like the more general term, while sum is primarily, (not exclusively) used for math. I hope this is a bit helpful. ^.^
You say what is the sum. Greek has different uses for ποιο and τι. Τι more asks for the nature of something, whereas ποιο is used in cases where "what" in english asks about something. So, τι είναι το άθροισμα (better translated as "what is sum") gets an answer like "the total amount resulting from the addition of two or more numbers, amounts, or items." and ποιο είναι το άθροισμα gets an answer like "2000".