In a RTS game, the player has to combat its enemies while running his war economy.
Everything run in so called real-time, in opposition to turn based games where the player always has to wait for his turn. The trick is to balance the effort put into building the economy and building and army to defend an attack the enemies.
One thing you will miss in Commander Stalin is the fact that there is no single-player campaign unfortunately.
The graphics are average and the units level of detail is pretty good so if you are into real time strategy games, you should give Commander Stalin a try.
Download Commander Stalin (Packages available for Windows, Linux and source files).