The marauder can only attack ground targets. Its attack is more powerful against armored units; it may slow units, except for massive units.
The marauder is an early-game support unit, useful for supporting marines by slowing fast melee units. It is effective against armored units, like the roach. They are also powerful against stalkers, and, to a lesser extent, siege tanks, immortals and thors. Marauders can destroy buildings very quickly. The marauder is vulnerable to zealots, marines, and zerglings. As a general rule, a player battling zerg should build fewer marauders than usual, while against the protoss, marauders are effective, though work well with medivac dropships as a "commando force" when battling the former.
The marauder's attack slows all units in target area.
Concussive Shells actually form a gravitational warp when they detonate. This warp not only damages the target, but it also slows the movement speed of all nearby units to a crawl for a short period of time. It does not slow down massive units.
The following section contains information from a previous version of StarCraft II which is no longer valid.
The idea behind the marauder was to act as a sort of middleground between marines and vikings—mechanized infantry, yet not an actual mech.
The marauder was originally equipped with dual concussion-grenade launchers which reduce a biological unit's speed immediately by a set percentage. A charging zealot would still move quickly after being struck by a grenade.
Prior to StarCraft II beta patch #8, Concussion Grenades was an automatic upgrade. It later became a researched ability.
The Kinetic Foam upgrade in the single player campaign was originally known as "Ablative Scales". The original design for the button was even completed but was never used in the final product.
↑Reapers are actually one of the best units against any light units in the game. They kill Zerglings, workers, and even Zealots pretty well with a little maneuvering. Their mines also decimate stationary defenses, as well as tech buildings. They are no doubt one of the best raiders in the game right now.
Marauders on the other hand are probably better mixed, unless you are going up against an all armored ground force. Marauders work well against both Protoss and Zerg. Early game, as mentioned above, they are great for slowing Zealots while Marines do the damage. Against Zerg, they are better suited against those pesky armored Roaches with the fast regen. Karune. 2008-10-21. Reapers and Marauders. Battle.net StarCraft II General Discussion Forum. Accessed 2008-10-21.