Octal is a base 8 numbering system, in which each digit of an octal number has 8 possible values, 0-7. Octal is useful in computers when representing values that fill three bit multiples. For example, Unix file permissions use octal numbering, because the actual binary data that represents file permissions are twelve bits long.
A traditional decimal number is the sum of digits multiplied by powers of ten. Octal is the same, but with powers of eight.
723518 to hexadecimal:
723518 = 7×84+2×83+3×82+5×81+1×80 = 2992910