When data that has been entered into HTML forms is submitted, the form field names and values are encoded and sent to the server in an HTTP request message using method GET or POST, or, historically, via email. The encoding used by default is based on a very early version of the general URI percent-encoding rules, with a number of modifications such as newline normalization and replacing spaces with "+" instead of "%20". The MIME type of data encoded this way is application/x-www-form-urlencoded, and it is currently defined (still in a very outdated manner) in the HTML and XForms specifications. In addition, the CGI specification contains rules for how web servers decode data of this type and make it available to applications.
A simple online tool that does exactly what it says; decodes URL encoding and encodes into it quickly and easily. URL encode your data in a hassle-free way, or decode it into human-readable format.