No Step Back is entirely about the Eastern Front: it overhauls the USSR and Poland, adds the three Baltic states, and along the way changes how land divisions get designed across the entire game.
What New Content It Adds
No Step Back splits its additions between several Eastern Front focus trees and a handful of system-wide mechanics that change how land divisions get designed across the whole game, not just in the East.
Nations: the USSR, Poland, and the Baltics
The 
USSR and 
Poland get completely overhauled focus trees, far deeper than the base game versions. Estonia, Latvia and Lithuania stop being set dressing and get playable content of their own.
Mechanics: the Officer Corps and Tank Designer
What people actually remember this DLC for is the mechanics: the officer corps (command points split between generals and army-wide bonuses), army spirits (national traits reflecting each country's military culture), a modular tank designer as deep as the naval and aircraft ones, and railway guns as a new heavy artillery piece. The tank designer in particular changed how armor gets built — no more picking from a handful of fixed models, you compose the tank piece by piece, with the same trade-offs that already existed for ships and planes.
Who's It For?
If you're playing the European theater — the USSR, Germany, or any power that ends up on the Eastern Front — the officer corps and the tank designer show up in almost any long game. It's one of the expansions with the most cross-cutting mechanics in the whole catalog, not just country content.