What is an SIC type rating? 14 CFR 61.55 says:


  ...
  
  (d) A person may receive a second-in-command pilot type rating for an
  aircraft after satisfactorily completing the second-in-command
  familiarization training requirements under paragraph (b) of this
  section in that type of aircraft provided the training was completed
  within the 12 calendar months before the month of application for the
  SIC pilot type rating. The person must comply with the following
  application and pilot certification procedures:
  
  ...
  
  (6) The applicant must appear in person at a FAA Flight Standards
  District Office or to an Examiner with his or her logbook/training
  records and with the completed and signed FAA Form 8710-1.
  
  (7) There is no practical test required for the issuance of the “SIC
  Privileges Only” pilot type rating.


What exactly is an SIC type rating used for and how can someone get a "type rating" without any kind of practical test?
It's an ICAO requirement, basically it's just official documentation showing that you've been trained and are  competent to act as SIC in a given airplane.

ICAO is an organization that standardizes regulations and procedures among the participating countries (close to 200 if I recall).  If you fly as SIC internationally you'll probably need an SIC type rating.

The rating is almost the same as a PIC type rating, except the rejected takeoff and taxi requirements (and maybe some others) don't have to be met.  It's basically a formality for any airline first officer as they will have met all the requirements just by going through the airline's training.