Paste text and click a button. Base64 encode turns text (including emoji and non-English characters, via UTF-8) into a Base64 string; Base64 decode reverses it. URL encode escapes characters such as spaces, & and ? so text is safe inside a link's query string; URL decode restores it.
A way to represent binary or text data using only 64 safe ASCII characters (A-Z, a-z, 0-9, + and /). It is used in emails, data URLs and APIs. It is encoding, not encryption.
No, everything happens in your browser.
The input is not valid Base64 (or not valid UTF-8 text once decoded). Check for missing characters or stray spaces.