The term effectiveUser is used in much of the WeBWorK code to represent the "targeted user" -- the person that the page is being created for.
The user (or User) is the person actually viewing the page.
The user and effectiveUser are frequently the same.
When a professor "acts as" a student however the effective (targeted) user is the student and the user is the instructor viewing the page.
This issue is important when deciding permissions and which items are to be shown. Should the decision be made on the basis of the viewer (user) or the targeted user (effectiveUser) - the one the page is being developed for?