The goal of continuous integration is to eliminate integration hell by catching bugs early.
For that end, a CI solution regularly merges, builds and tests source code. This allows to catch bugs early in the development cycle when they're easier and faster to fix.
XP: use in combination with automated test scripts.
Artifacts that are produced by continues integration are deployed to test and production environments with continuous delivery.
Distinguish from frequent integration.
A build server runs the unit tests automatically and periodically or after a commit. It returns the result to the developer.