ROUND evaluates the number argument and rounds it to the specified number of decimal places using standard half-up rounding: if the digit immediately after the last kept decimal place is 5 or higher, the last kept digit rounds up; if it is 4 or lower, it stays the same. The result is stored as the rounded value, not just displayed that way.
ROUNDUP works identically except it always rounds away from zero regardless of the following digit -- even 12.341 becomes 12.35 with ROUNDUP(x,2).
ROUNDDOWN always truncates toward zero -- 12.349 becomes 12.34. For currency work, ROUND(value,2) is the correct default. ROUNDUP is used when a fractional penny must always become a full penny (common in tax and VAT calculations). ROUNDDOWN is used when a partial unit should never be counted as a full unit. Negative num_digits rounds to the left of the decimal: ROUND(1567,-2) returns 1600.