Terminology

Rewritten-docs.md

@@ -1800,14 +1800,20 @@ 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.
+**Exercise** -- An algorithmic problem that can be assigned to a group.  They
-They can be shared by the teachers using an exercise database in ReCodEx.
+ can be shared by the teachers using an exercise database in ReCodEx.
 
 **Assignment** -- An exercise assigned to a group, possibly with modifications.
 
-**Runtime environment** -- @todo
+**Runtime environment** -- Runtime environment is unique combination of platform
+ (OS) and programming language runtime/compiler in specific version. Runtime
+ environments are managed by the administrators to reflect abilities of whole
+ system.
 
-**Hardware group** -- @todo
+**Hardware group** -- Hardware group is a set of workers with similar hardware.
+ Its purpose is to group workers that are likely to run a program using the same
+ amount of resources. Hardware groups are managed byt the system administrators
+ who have to keep them up-to-date.
 
 ## General basics