There are many types of instructor rating and this is quite a complex area. We have detailed a few of the most useful ones and will update the list as we go.
Class Rating Instructor (CRI) - Real benefit is it can be added to a PPL. Useful for a club check pilot or for someone that wants to put something back into aviation. You can't instruct ab initio PPL students with a CRI. You can fly the 2 yearly instructional flight that PPL holders require and useful for checking out pilots to a new type within the class.
Flying Instructor (FI) - Initially restricted but allows you to instruct as above but you can now teach new PPL students. The restriction is you can't send someone solo. This restriction is lifted with hours and experience. You can also have the restrictions removed for other ratings such as aerobatics and night. You can recieve money for instruction because you must hold a CPL.
Select the instructor rating you want more info on: