It should be noted that in regards to classes ONLY the Black Templar Space Marine class is confirmed as affiliated with Black Templar. The other classes of the Imperium of Man are not necessarily Black Templar affiliated.
The classes system seems to be broken down into 5 archetypes. It can be inferred that each has a distinct role.
Assumed Classes from the Space Marine Trailer are depicted on the picture to the left.
An interview with David Adams confirms the class of fulfilling the Melee role. From what we've seen it looks like the Bolter and Power Sword will be the initial weapons of the Black Templar Space Marine.
It is believed the Tech-priest will fulfilling the role of being a mechanic/support type. The THQ/Relic game Dawn of War featured Tech-priests as the construction unit for the Imperial Guard. From what we've seen it looks like the Lasrifle, Power Axe will be the initial weapons of the Tech-priest.
Light Armour and a Sniper Rifle.
The class in the white robes has been seen to use physic warding. Fans speculate this role may be a Inquisitor or Librarian. More accurate observation suggests this class is a sanctioned Psyker of the Adeptus Astra Telepathica, a Primaris Psyker.
The Black Templar do not include Psykers in their midst. Contrary to what some fans think this class will not be definitely affiliated with them since Imperial Pyskers are not Space Marines.
Little of him has been seen. All we know is that the class will start with a Shotgun.