There are several patterns (or "marks") of Power Armour, whose appearances differ significantly.
Many older marks have special associations for certain Chapters and are often worn by their ceremonial Honour Guards or elite units.
It also commonly includes numerous auxiliary systems such as radio frequency communicators, auto-senses, etc.
Space Marines go through an arduous process where they receive various biogenetically-engineered implants culminating with the implantation of the Black Carapace which allows the Astartes a direct and instinctual neural interface with his Power Armour, transforming it into effectively a second skin.
Working examples of the older Marks, meanwhile, are extremely rare and valued as a physical link to the glorious history of the Space Marines.
It was used during the Emperor's campaign to retake and reunite the Sol System before the Great Crusade.
It was by no means unique to the first Space Marines -- in fact, the techno-barbarian warriors the Emperor fought during the conquest of Terra wore it too, and the Emperor's forces of genetically-enhanced warriors were wearing it even before the first true Space Marines were developed.
Power Armour is an advanced form of powered combat armour, worn primarily by the Space Marines and the Chaos Space Marines, though suits have been created to be worn by mere mortals.
It is a completely enclosed suit of combat armour composed of shaped Adamantium and Plasteel plates, encased in a Ceramite ablative layer.