Sakai as a produce is based on a layered service architecture. If we focus on these services, they break down into the following layers:
Application Services - services tied to a particular application, like chat.
Educational Services - services that support education like course management and gradebook.
Common Services - basic services widely used like user, group, site, authorization.
Framework Services - core services that may require persistence, like entities.
Kernel Services - core services that do not require persistence, like components, sessions, tools, etc.
This stack of services largely reflects the architecture presently in Sakai 2.1, but also reflects thinking moving towards 2.2 and the future, particularly around the definition of Framework Services.