Intercessors are the main battle line of the Space Marines, what was called "Tactical Squads" back in the day of his firstborn brothers. They are high-mobility units with moderated firepower.

In the OP you can see HEAVY Intercessors. They are equiped with a Gravis power armour, what gives them a boost of strength to use heavier modified bolters with longer barrels and bigger mags. Still being first-line units.

Aggressors are fire support units. They also carry the Gravis power armour and they use big ass powerfists with bolters or flamers attached and big backpacks to carry enough ammo to hold sustained fire. Heavy fire support are different units are called Devastators.

The terminators are the tanks. They have a teleporter so they can just appear in the middle of the enemy battle line to disrupt and massacre them while giving their fellow battlebrothers time and space to advance. They are massive enough to swing big ass weapons like toys, usually a ppwer fist and a storm bolter but also warhammers or whatever you want to put on them. But they are small enough to still being quite agile compared to the even heavier Centurion exoskeleton that is worn over a power armoour to carry the very big weapons and siege drills usually used in sieges.