I am not a biologist, but the way I was taught was that monkeys have tails and apes do not.

As far as the spelling, "monkeys" is correct.

You may be thinking that you want the plural of monkey, but because it ends in y the ending should become -ies. For example: berry -> berries

However, that rule is a little more complicated, and the ending of monkey is -ey. Because there is a vowel before the y the ending you don't have to change the -y to -ie and instead simply add -s

English is stupid.