A “NoSteam” (or “revEmu”) install is a cracked/pirated version that bypasses Valve’s authentication servers. It allows the user to:

When you booted up build 1.35.2.2, the first thing you saw was the fiery red and black interface of the pass. This was the era when the community was obsessed with the return of a legend: the re-imagined Nuke . It had just been overhauled with a clean, nuclear-white aesthetic, and players were still arguing over whether the new "vent" sounds and the updated "A" site layout were better than the original. The Gameplay Experience

It retains the original UI (Scaleform or early Panorama) and weapon handling that many veterans prefer. Technical Breakdown