There are often confusions in terminologies between "certification" versus "rating." In aviation, a certificate confers a privilege (not a right) from the FAA. The certificate defines what you can do as a pilot; It is an indicator that a certain level of skill has been achieved.
Sports, recreational and private pilots, for example, are not legally allowed to fly for pay.
Those with Commercial or Certified Flight Instructor (CFI) certifications can.
However, insurance company restrictions are generally much stricter than those of the FAA. Since insurances are such an important part of deciding who flies, for the purposes of this blog post, let's look at some of the certifications and the rules governed by what aircraft pilots fly.