Razorback Transport
The Razorback was developed after the Horus Heresy, and was intended to provide Space Marines with heavier fire-support. By replacing some of the Rhino Transport's crew and cargo capacity with ammunition storage and weapon hardpoints, the new transport vehicle becomes capable of bringing troops into the battle, and providing amicable close-support once there. The notably-reduced crew capacity means that the Razorback can only transport smaller squads into battle, and the Razorback loses its ability to allow troops to fire out - but the heavy weapons it adds to the mix makes for a very cost-effective way to get stronger guns on the table for cheap, whilst still providing transport.
The Chaos Space Marines were intrigued by the design after encountering it, but ultimately decided to simply arm their Rhinos with support weapons instead, since that didn't involve flagging down manufacturing plants (of which they don't have anywhere near as much as the loyalists) or cutting the transport capacity by 40%. as such, it remains unique to the Space Marines.
Overview
Razorbacks are kind of an oddity. They have very minimal transport ability (6 models compared to the 10 of a typical Rhino) and they cost a bit more (considerably more depending on the weapon option). They also lose out on firepoints, and are a much more prominent target than a regular Rhino, since their heavy weapon basically announces their threat level to everyone on the battlefield.
On the other hand, the fact that they have said mounted heavy weapon - and one worthy of a much heavier vehicle - makes them quite useful. They are, perhaps, best-suited for use by Sternguard Veterans or Devastators, who can use the Razorback to quickly re-deploy and benefit from its ability to unload heavy weapons in support of a squad.
Razorbacks by default come with a Twin-Linked Heavy Bolter, but they can back this up with a Twin-Linked Flamer, Twin-Linked Assault Cannon, Twin-Linked Lascannon, or a combination weapon that integrates a Lascannon and a Twin-Linked Plasma Gun. Heavy Flamer Razorbacks can be useful for escorting Melta suicide squads, but the others are all better-used for supporting Devastators and the like. Tank-Busting squads benefit from a Lascannon Razorback, whereas Infantry squads often do well with just the basic Heavy Bolter Razorback. The Assault Cannon Razorback is a good comprimise of power and strength.
Regardless of which you use, make sure that you're careful with them - they cost notably more than a Rhino (or Chaos Rhino w/Havoc Launcher for that matter) and you can't simply use them as expendable transports like you can with their little metal boxxy brothers - they cost too much for that. Used carefully, however, they are a godsend to any force that likes it some heavy weapons.