terminology

Rewritten-docs.md

@@ -1728,8 +1728,28 @@ applications.
 
 ## Terminology
 
-@todo: Describe the terminology: Instance, User, Group, Student,
-Supervisor, Admin
+**Instance** -- Represents a university, company or some other organizational
+unit. Multiple instances can exist in a single ReCodEx installation.
+
+**Group** -- A group of students to which exercises are assigned by a
+supervisor. It should typically correspond with a real world lab group.
+
+**User** -- A person that interacts with the system using the web interface (or
+an alternative client).
+
+**Student** -- A user who is subscribed to some groups and submits solutions to
+exercise assignments.
+
+**Supervisor** -- A person responsible for assigning exercises to a group and
+reviewing submissions.
+
+**Admin** -- A person responsible for the maintenance of the system and fixing
+problems supervisors cannot solve.
+
+**Exercise** -- An algorithmic problem that can be assigned to a group.
+They can be shared by the teachers using an exercise database in ReCodEx.
+
+**Assignment** -- An exercise assigned to a group, possibly with modifications.
 
 ## General basics