ASCAP, The American Society of Composers, Authors, and Publishers, has a list of FAQs that provide additional useful information for business owners and other music users. You can also see the full list of ASCAP license types here.
BMI, Broadcast Music, Inc, also has a list of FAQs or you can search their license types here.
SESAC's list of music license types can be found here and their FAQs here.
More about GMR and their licensing FAQ can be found here.