There are 5 levels of the CMM. According to the SEI, "Predictability, effectiveness, and control of an organization's software processes are believed to improve as the organization moves up these five levels. While not rigorous, the empirical evidence to date supports this belief."
1.    Initial: Software development follows little to no rules. The project may go from one crisis to the next. The success of the project depends on the skills of individual developers. They may need to finish the project in an heroic effort.
2.    Repeatable: Software development successes are repeatable. The organization may use some basic project management to track cost and schedule. The precise implementation differs from project to project within the organisation.
3.    Defined: Software development across the organisation uses the same rules and events for project management. Crucially, the organization follows this process even under schedule pressures, ideally because management recognizes that it is the fastest way to finish.
4.    Managed: Using precise measurements, management can effectively control the software development effort. In particular, management can identify ways to adjust and adapt the process to particular projects without measurable losses of quality or deviations from specifications.
5.    Optimizing: Quantitative feedback from previous projects is used to improve the project management, usually using pilot projects, using the skills shown in level 4.