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 is 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 re-positioning 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.
Grey Knights Strike Cruiser
Strike Cruisers of the Grey Knights are noted to be more advanced than 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.
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. Even at the relatively small numbers, a dozen or so per sub sector is more than enough to ruin any pirates/xeno raiders day while being able to delay larger fleets for the Navy to respond.
The Castigator class of the Calixis sector is a probable variant of the Punisher, lacking the torpedo tubes but otherwise serving the same role in the sector Arbites.
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. Though it can equip a second shield it rarely helps, unless you have some squadron shenanigans in mind.
Funny enough, just as the larger version it boasts 6+ armour on every side, and can basically laugh off battleship-size battery shots when showing it's side (yeah, GeeDubs thinks shooting at exposed broadside is much much more difficult than aiming at an enemy running away or charging prow-in (it actually is, when counting in ranges and lead, Battlefleet is 'played')).
So while being a light cruiser in terms of hit points and shields, it simply laughs at batteries, torpedoes and bombers, and if a squadron of these "light cruisers" does get into range, BOY YOU BETTER LUBE UP because they will go in dry, first derping you with boarding parties from their thunderhawks (that are not just uber-fat fighters but also assault boats or bombers), then bring down your shields (and most likely get some crits on top because BOMBARDMENT CANNON thinks every foe is an elfdar ripe for critting).
And then they will teleport assault you for even more crits.