The definition of Build is provided in Artifact: Build, and can be summarized as "an operational version of a system or part of a system".
In a J2EE project, a system-level build is typically composed of a J2EE application (see Guideline: J2EE Application), but may also include:
A subsystem-level build is typically composed of one or more J2EE Modules, but may encapsulate databases and other
Builds are created by integrating components. In a J2EE environment, this typically means assembling J2EE Modules and
J2EE Applications. For more information, see Guideline: Assembling J2EE Modules and Guideline: Assembling J2EE Applications.