Build Agents are computers with installed TeamCity agent software capable of running different types of build scripts.
As developers integrate their work results into the Version Control System, TeamCity server distributes them to the Build Agents automatically.
Build Agents detect and pass to the server a wide range of software (like language interpreters and testing tools) and hardware (like CPU or disk space) parameters which can be utilized in build scripts.
Upon server upgrade or downgrade, Build Agents upgrade or downgrade themselves automatically.