Web Design in a Nutshell, 3rd Edition

Encoding

Another behind-the-scenes step that happens in the transaction is that the data gets encoded using standard URL encoding . This is a method for translating spaces and other characters not permitted in URLs (such as slashes) into their hexadecimal equivalents. For example, the space character translates to %20, and the slash character is transferred as %2F.

The default encoding format, the Internet Media Type (application/x-www-form-urlencoded), will suffice for most forms. If your form includes a file input type (for uploading documents to the server), you should use the enctype attribute to set the encoding to its alternate setting, multipart/form-data.