So far I've described PLT Scheme's first-class component systems, as well as the second-class, static module system. Of course, Scheme is a reflective language. Where there's eval, there's not just one "compile time." Code can be compiled and evaluated at any time during the execution of a PLT Scheme virtual environment.
Moreover, considering that one of the most important applications written in PLT Scheme--DrScheme--is an IDE, it's clearly important to be able to load and reload a single module multiple times. Users have to be able to edit a module and re-evaluate it without restarting the Scheme image. (In fact, it's even possible to run DrScheme from DrScheme, so it must even be possible to have multiple instances of the same module running concurrently!)
Every instance of a module in a running PLT Scheme environment has associated with it a namespace. Namespaces serve two somewhat orthogonal purposes. The first is to store top-level bindings. A module's namespace contains the mapping of its global definitions (some of which may be made available to other modules via provide). There is also a namespace associated with the REPL, which can be dynamically modified; in DrScheme, that namespace is typically just the one associated with the current running instance of the module being edited. In a sense, this role of namespaces is to mediate between the static, segmented architecture of modules and the dynamic, global nature of the Scheme top-level. This is an interesting tension that comes in whenever designing a lexically scoped language with dynamic code evaluation such as with eval or a REPL.
The second purpose of namespaces is to maintain an internal module registry, which keeps track of running instances of modules. Every namespace has its own registry, but registries may share instances. This is how it's possible both to create multiple, isolated instances of modules and to share module instances and their associated state.