One step, both ways
ROT13 moves every letter 13 places along the alphabet. The alphabet has 26 letters, so doing it twice brings you back where you started: Hello becomes Uryyb, and Uryyb becomes Hello. That is why there is no setting and no difference between encoding and decoding; type in either box.
Where it is used
ROT13 hides text from a casual glance rather than from anyone trying: spoilers, puzzle answers and punchlines on forums and in old Usenet posts, so nobody reads them by accident. Only the letters A to Z change; digits, accented letters and punctuation pass through untouched.
Questions
Does it change numbers?
No. ROT5, for digits, and ROT47, for every printable ASCII character, are relatives that also move numbers and symbols; they are not on this page.
Is ROT13 a Caesar cipher?
Yes, a Caesar shift of exactly 13. The Caesar Cipher page does every other shift.