The Marauder is a tier 1.5 mercenary. It is built for killing armored units. At the barracks where it is spawned, you can purchase Concussive Shells for 150, and Maraiders for 600. Maraiders change the unit model and portrait to "KillSquad" Marauders.
The Maraider has the ability to shoot air units, has higher health, armor, biological damage, and splash damage/radius.
Consequently, it loses most of its armored damage and some attack speed, and increases its bounty and rebuild costs.
Teching to Maraider is not reverse able.
Techning to Maraider usually is only a smart idea if there are air units present in the map, or you require that extra splash damage/radius to fight back late game Teched Marines. It is not a good idea to tech straight to Maraider, especially if there are Marauders and Stalkers present on the map.