Build agent represents the QuickBuild instance configured to run as agent. It must be connected to a running build server to join a build grid, and it can be upgraded automatically when the build server upgrades to a new version. As a member of the build grid, it executes steps sent to it from the build server.
Build agents connect to build server to form a build grid. Since a grid node (server node and agent node) can only run one build at a time, multiple build agents means the ability to run multiple builds concurrently, which is important to a busy build system. It also make multi-platform build/test possible as a build can distribute different steps to different agents for execution. Build agent can be upgraded automatically when the build server upgrades to a new version.