There are a number of ways that resources in the Workbench can be built. The scope of a build can be one or more selected projects, a working set, or the entire workspace.
A build will typically only operate on resources that have changed since the last build. A clean build will discard all existing built state, causing the next build to operate on all resources within the scope of the build.
Builds can be done automatically (each time resources are modified), or manually, using a menu item or keyboard shortcut. See the Related tasks links for more details.