So I hope this overview has given you a more precise idea of what Space Marine is : a small scale sci fi wargame where two combat sections composed of 20 men meet you have to throw 40 D100s to know who hits and then as many D10 to know who gets through to do damage. You can find this system a bit heavy at that level but those who like Warhammer will not be on unfamiliar territory. Anyway, the game benefits from assets borrowed from its big brother (Space Opera if you didn't know) - ie it is very well deocumented and you can find a comprehensive overview of sci fi.