Legions of Steel is in the same style as Space Hulk - while this is a full description, it does include comparisons to Space Hulk to help understand the differences. LOS features a modular board of passageways and rooms, doors, dice, and miniatures. The rule book has several scenarios.
It is a squad-based tactical game. One player plays the commandos (typically two squads of 4) and the other player plays the machines (typically one "Mark III Assault Fiend" and a horde of 10 nightmares - skeleton like robots). Whereas in Space Hulk where the marines are slow but have ranged weapons, in LOS everyone has ranged weapons. But the commandos are more accurate, particularly at close range. They have leadership points, similar to Space Hulk, and one heavy weapons trooper with a short range but double-die plasma weapon. The Assault Fiend, for the machine player, is an exception. He moves quickly, takes two hits, and only has short range weapons, but is very deadly. To add an additional differentiation each unit has grenades. Each commando has a force wall grenade (blocks one tile), and a K-pulse grenade (like a frag but low chance of success). The nightmares use "nachtmacher's" which are like super smoke grenades - they create an area of pitch darkness.
While the basic mechanics are similar to Space Hulk - move and shoot, one hit kills - there are some notable differences. Space Hulk, particularly in the later edition, uses a basic rule set. Other than move and shoot, you can also put a marine into "Cover Fire" which allows you to take shots at any target that enter's your line of sight. In Legions of Steel the rules are slightly more complex. For example, there are several battletech-ish modifiers to be aware of, such as firing through corners or firing after running. You have "Cover Fire" which is simply delaying your firing action to your opponent's turn - and so you only get one shot. You also have "Suppression Fire" which is more like the Cover Fire in Space Hulk, except it is a constant stream of fire, no risk of jamming, and more effective. You also have more fire options, such as auto-fire vs burst fire (a basic mathematical evaluation usually identifies a clear favorite).
The basic rulebook includes several scenarios. Each scenario details how to build the map with the given tiles, the starting conditions, and objectives. Objectives are things like deliver the bomb, escape, kill, defend, search.. While there are no rules for custom maps or even campaign play, there is certainly no limit to your creativity. For example, having a hidden traitor unit on the commandos team, playing a double-blind game (two seperated boards and one game master), etc.
Technically, it is a 2 player game. However, it is easy and enjoyable to play with 3 with the commando squads divided.