Base64 is a type of encoding that allows you to place any type of data that might have special characters in it (which would cause errors in parsing) into the code as just numbers and letters.
http://en.wikipedia.org/wiki/Base64
The sequence, data:image/png;base64, is a key that tells the browser the following string contains an encoded version of an image. Instead of loading the image from a remote URL, it is included right into the CSS page. The browser will decode that string into the image file. http://en.wikipedia.org/wiki/Data_URI_scheme
