Continuous Integration (CI) is a development process that requires developers to integrate their code into a shared source code repository (git,svn,mercurial,etc) several times a day, while a monitoring process detects code commits and acts upon those commits. Those actions can be the actual checkout of branches, execution of build processes, quality control, and of course our favorite; automated testing.
Decrease the feedback loop
Discover defects faster before production releases
Increase code visibility and promote communication
Increase quality control
Reduce integration issues with other features
Develop in an Agile/Scrum fashion with continuous improvement
Here is a listing of some of the major CI servers: