C.J.,
The commercial certificate (there is no such thing as a commercial license for pilots in the United States) is essentially a glorified private pilot certificate, with a few more privileges. It allows an individual to carry persons or property for compensation or hire. Otherwise, the privileges are no different.
The commercial certificate is restricted to flying more than 50 nm, or at night for hre, without an instrument rating.
As the previous posters stated, the commercial certificate is a prerequisite for obtaining the Flight Instructor certificate. There is only one instructor certificate: there is no such thing as a CFII or MEI. The instructor certificate has ratings which define the teaching privileges; the instrument, and multi-engine ratings. However, these are not separate certificates, but simply ratings on the flight instructor certificate.
A Ground Instructor certificate has no pre-requisites. One need not hold a pilot certificate of any kind, or a medical certificate. There are no practical tests, and one need not have an instructor's endorsement to take the knowledge exam ("written" test). One need only take the Fundamentals of Instructing exam, and one of three different ground instructor 'written' tests to qualify for the certificate. Application is made directly to the FSDO (flight standards district office), and the temporary certificate issued in person.
The ground instructor certificate will enable you to teach ground subjects required for the issuance of a certificate or rating, and will enable you to recommend applicants for a knowledge exam. It also enables you to endorse logbooks for instruction given, and this instruction may be credited toward a certificate or rating, as appropriate.