The armor of the Astartes is the most advanced piece of machinery that mankind has ever produced. Mixing the most micro-sized machinery and barely-understood biomechanical processes, the armor provides the warrior with additional strength and speed on top of the survivability.
The interior machinery of the armor is made up of thousands of hyper-servos and mechandritic branches. The armor weighs hundreds of pounds, which would be impossible to move by an un-augmented human. The Astartes physiology makes them stronger by many times, allowing them to move the armor with little effort. This is further enhanced by the psycho-active matrix of the power armor, which activates the machinery and moves the heavy armor plates with only thought. Added to the space marine's already unimaginable strength, this allows the armor to act as additional muscles. It is not unheard of for a space marine to be able to throw a tank using nothing but his hands, and power armor.
The entire suit of power armor is controlled by a miniature fusion reactor housed in an armored backpack. Due to the intricate and super-charged machinery inside and the heavy weight of the armor plates and Astartes warrior enclosed in it, the suit requires a massive amount of energy. This can only be provided by nuclear forces, and fusion is the simplest and most transportable form. The energy produced by the mini-reactor is enough to power the suit forever, but the heat would fry a man in the suit instanty. This is lessoned almost entirely by massive heat dispersing modules on either side of the backpack.
Power armor is the most advanced battlefield asset in the entire galaxy. In addition to protecting and enhancing the warrior's strength and speed, it also contains the wargear necessary for space marine shock troops to wage their wars. Housed in the chestplate is the master power trunk, carrying enough power to sustain an Imperial tank. Also housed there is nutrient-producing injection machinary, used to physically sustain the soldier without having to stop for food. The leg armor contains additional gyrostabilizers and gravity enhancers to allow for extremely steady footing. The feet have magnetized and grounded footpads, perfect for boarding actions and interstellar combat. More advanced than all of this, however, is the helmet. The smallest piece of the entire Power Armor suit, it is nonetheless the most packed with machinery. The helmet of most space marines contains vox-systems, heads-up-display capability, orbital and transatmospheric vox-systems, voice amplifiers, air filtration and air-sealed environmental systems, multi-spectral vision capability, and audible enhancers. With all this wargear, an Astartes is able to wage war at night, in extremely hostile environments, and against grenades and chemical weapons.
Even more advanced, so advanced that the Adeptus Mechanicus guards the secret closely, is the biomatrix. The armor has a number of 'spikes' that plug into a space marine's implanted Black Carapace and 'reads' the neurons underneath. In effect, the power armor's matrix turns the space marine's brain activity into electronic signals which power the armor. This allows an Astartes to control the armor with his thoughts in the same way as he controls his own skin and muscles. What's more amazing is that the warrior doesn't even need to try to control the armor, it moves the same way as his arm when he wishes to move it. With the space marine's great strength, speed, and skill being augmented by the power armor, the Astartes become more than any other warrior in the galaxy can match.
The only technical drawing I've seen of power armor is from the Rogue Trader days and is of the Mk 6 Corvus armor. Check out how packed the helmet is, and pay especially close attention to the psy-spikes in the back of the chestplate. That's how the power armor is told what to do by the marine's brain...
The Space Marine Power Armor has grown over 10 milennia of war across the stars. Although the basic characteristics and purposes remain the same, key advancements have been made with each upgrade, or Mark (Mk).
The Emporer sanctioned the use of this armor at around the same time as the launch of the 'Great Crusade'. As his armies went out and conquered the nearby star systems, they needed a fully enclosed suit of armor that they could also fight in. Sadly, the armor provided very low mobity, but great protection. Some feel that Tactical Dreadnought Armor began its development here.
Mk VII 'Aquila' or 'Imperator' armor