Space Marine Strike Cruiser
A Strike Cruiser is a class of warship used by Space Marine Chapters. They are the workhorses of an Astartes fleet, and are often seen spearheading the planetary assaults that the Space Marines are famous for.
The Emperor's Scalpel
Strike Cruisers are essentially scaled-down versions of Battle-barges, designed to transport and support a Battle Company's worth of Astartes and their equipment. Because it iss not as armored or shielded as a Battle Barge, the Strike Cruiser has to rely instead on its speed to get its complement of troops to wherever they are needed.
Once at a hotspot, a Strike Cruiser can then quickly deploy its Space Marines through various means -- drop pod, thunderhawk, or even teleportation -- before repositioning to another part of orbit, ready to retrieve the Marines on-world or provide other means of support. Said support often involves Orbital Bombardment, which thanks to its armaments the Strike Cruiser is well-suited to.
Variants
Grey Knights Strike Cruiser
Strike Cruisers of the Grey Knights are noted to be more advanced that anything fielded by their contemporaries, being faster, tougher, and heavier armed. Not only that, but their hulls are shielded against daemonic influence, and are piloted by the finest Navigators available. If their existence was more widely known, these ships would be the envy of the greater Imperium.
Thanks to all these enhancements, the Grey Knights can respond to any Daemonic incursion well in advance of any other Imperial force.
Punisher
The Punisher is variant used by the Adeptus Arbites. Primary a patrol vessel, the Punisher class features less armor and heavy weaponry compared to Astartes Strike Cruisers. In return it gets a specialized scanning array used to locate pirate strongholds or heretic bases on the frontier.
Because of the rarity of Arbites ships in general, the Punishers don't normally engage in fleet actions, though it is more than capable of defending itself thanks to its torpedo tubes, attack craft, and Bombardment Cannon.
Vanguard
Introduced in Battlefleet Gothic: Armada, the Vanguard is a modified Strike Cruiser, trading offensive power for better thrusters and point-defense turrets. This makes it less effective in planetary assaults, but viable as either scouts or heavy fleet escorts.
The Vanguard itself comes in three flavors, depending on whether it gets armed with torpedo tubes, lances, or the Bombardment Cannon to complement its macrocannons and launch bays (the last configuration matches the "classic" Strike Cruiser more commonly known in fluff).
In Battlefleet Gothic
The Strike Cruiser is a Light Cruiser, and equivalent to the Dauntless-class used by the Imperial Navy. Due to its lack of Lance batteries, the class is less effective at long-range battles, and have to close in to get most of its damage done.
Its standard armament is two weapons batteries, a flight deck capable of launching two Thunderhawk Gunship squadrons, and the Bombardment Cannon.