In most cases, CMOs are not-for-profit entities. They manage music rights for authors, performers, producers and other right holders to simplify the management of those rights. Some of the services they offer:

• monitor the use of when, where and what works are used;
• negotiate tariffs and other conditions with users;
• license the use of protected works on behalf of its members and of other rights holders it represents; and
• collect the fees from users and distribute these to the rights holders.

Depending on the repertoire they represent, they can also be called: 

• Music Licensing Companies (MLCs)
• Mechanical Rights Organizations (MROs)
• Performers’ Collective Management Organizations (PMOs)
• Reproduction Rights Organizations (RROs)

source: WIPO