Continuous integration is a term used in Agile development project to denote the fact that integration of all the code base is done as frequently as possible. That means getting the latest version from source control and building it all. It also means running all the unit tests on the latest build. This early integration allows the team to catch bugs and bad code as early as possible, when it's still easy and quick to find and fix. CruiseControl.Net is one of the best known CI daemons. I suggest you give it a go on your own projects.