ROT13 is its own inverse, so Encode and Decode produce the identical result, running it twice always returns the original text.
ROT13 is a simple letter-substitution cipher, a special case of the Caesar cipher that shifts every letter exactly 13 places through the alphabet, wrapping from Z back to A. Because the alphabet has 26 letters, shifting by 13 twice returns every letter to its original position, ROT13 is mathematically self-inverse, so the same operation both encodes and decodes. It's not meant for real security; historically it was used on forums and newsgroups to hide spoilers, punchlines, or puzzle answers from casual glances while staying trivially reversible for anyone who wanted to read it.
Shifting by 13 twice equals shifting by 26, which is a full loop back to the starting letter of a 26-letter alphabet. That makes ROT13 self-inverse: applying the identical shift function a second time always restores the original text.
No. Only the 52 uppercase and lowercase Latin letters are shifted; digits, punctuation, whitespace, and any other characters pass through completely unchanged.
No, ROT13 provides no real security since it has no key and can be reversed instantly by anyone who recognizes it. It's intended purely as an obfuscation convention, not encryption.