Purpose
A Portal Context specifies major aspects of portal behavior: request processing, and presentation construction. uPortal installation can contain multiple portal contexts, which may be completely independent or sharing some of the components. Context is defined by IContext interface (see UML diagram below, pdf). CompoundContext explicitly separates request processing and content rendering into IRequestParameterProcessor and IPortalRenderer interfaces.
Context resolvers
In processing a request, uPortal chooses appropriate portal contexts with help of a context resolver (defined by IContextResolver interface). ServletPathContextResolver chooses the context based on the first element of the servlet path (i.e. "myContext" in http://host/uPortalApp/myContext/something/other)