A URL decoder decodes URL encoded data into a human-readable format. A URL is composed from a very limited set of characters. These characters include digits, letters and a few other special characters. Any other 'unsafe characters' cannot be put into a URL. URL encoding is used to pass these disallowed characters into a URL by changing them into a 'URL Safe' format. For example, one of the most commonly encoded characters is a space which has an ASCII value of 32. When you convert 32 to hex it equals 20, which is preceded by a '%' to give us the URL encoded value, %20.