The Space Marines are enhanced human beings with new organs placed into them that make them better fighters than a normal human. Space Marines are divided into Chapters, with approximately 1,000 members when the Chapter is at full strength. Chaos Space Marines have unlimited numbers - they are Legions, not Chapters. The Space Marines are called in during an extreme emergency.
One could make a comparison between the Space Marines and various knightly orders in medieval Europe. The Black Templars Chapter bears more than a passing resemblance to the late Knights Templar, an order of Christian Knights that fought in the crusades.
There is a large fanbase for the Space Marines. The good points are that they are tough individually, but they are expensive points-wise. Other than that, they generally have a good story base, etc. Quite a few of the more popular Chapters are close-combat oriented, as close combat can make or break a game in Warhammer 40,000 (enjoyment to some, a flaw to others).