Exercises described

Rewritten-docs.md

@@ -499,7 +499,7 @@ is to allow anyone join the group with supplementary confirmation of
 supervisors. It has no additional benefits, so approach with public and private
 groups is implemented.
 
-Supervisors using CodEx in their labs usually set minimum amout of points
+Supervisors using CodEx in their labs usually set minimum amount of points
 required to get a credit. These points can be get by solving assigned exercises.
 To visualy show users if they already have enough points, ReCodEx groups
 supports setting this limit. There are two equal ways how to set a limit --
@@ -521,11 +521,21 @@ allow users submit their solutions. License is granted for defined period of
 time and can be revoked in advance if the subject do not keep approved terms and
 conditions.
 
-@todo: explain instances why they are useful what they solve and also discuss licenses concept
-
-@todo: exercise, what is it why we need it
-
-@todo: explain why there is exercise and assignment division, what means what and how they are used
+The main work for the system is to evaluate programming exercises. The exercise
+is quite similar to homework assignment during school labs. When a homework is
+assigned, two things are important to know for users:
+
+- description of the problem
+- metadata -- when and whom to submit solutions, grading scale, penalties, etc.
+
+To reflect this idea teachers and students are already familiar with, we decided
+to keep separation between problem itself (_exercise_) and its _assignment_.
+Exercise only describes one problem and provides testing data with description
+of how to evaluate it. In fact, it is template for assignments.  Assignment then
+contains data from its exercise and additional metadata, which can be different
+for every assignment of the same exercise. This separation is natural for all
+users, in CodEx it is implemented in similar way and no other considerable
+solution was found.
 
 ### Evaluation unit executed by ReCodEx