This is a very standard format to represent combined date and time, known as ISO 8601. If you're parsing the string, the "T" separates the date and time. The Z indicates "zulu time", also known as UTC or Greenwich Mean Time (versus some other time zone).
You shouldn't have to parse the string yourself though - basically every popular programming language has the ability to take this string and turn it into a Date/Time representation appropriate to the language.
javascript:
date = new Date("2013-09-25T03:55:22.054Z");
ruby:
date = DateTime.iso8601("2013-09-25T03:55:22.054Z");
etc.