Scout

In Space Marine Chapters that follow the Codex Astartes, Scouts are Space Marines in training. They have not yet gained the Black Carapace, the gene-seed organ that allows a Marine to interface with his power armour. They are usually organized into squads in the Tenth Company, and are the only Marines capable of wielding sniper rifles or using camouflage. Fittingly, the Raven Guard use a lot of Scouts, since their combat doctrine centers around infiltration and sabotage.

They are lead by a Sergeant, who is usually a member of a Tactical Squad that chooses to impart his wisdom on a younger generation, but may also be a Veteran Scout who refused the opportunity to become a full-fledged Space Marine. In the case of the Blood Ravens, they are led by Steve Blum voicing an angsting, ineffectual dork, and he hopes one day to teach all the Blood Ravens to be angsting ineffectual dorks. Among his most important lessons is that, when facing Tyranids, the greatest tactical decision is to give up all hope kill the greatest threat. Aside from that, go out of the way to point out problems and question every solution as if it was the most inexperienced idea you've ever heard.

Also, if you even say the word "bonk" within a hundred feet of an Angry Marine Scout with a Power Bat, he will beat you so hard you won't know which end of your sternum you're looking through.

Non-Codex Chapters

Space Wolves take the opposite tactic: their Scouts are chosen from Tactical Marines who are experienced, skilled in tracking, and loners who don't play with others.

Black Templars do not use Scouts at all, instead mixing their Marines-in-training, which they call "Neophytes", into their normal battle-brother squads. They apparently think this gives the initiates better experience, but it's really just because they make amazing meat shields.