Here's a whole lot to take a look at:
Oh when the tripod be left home, what are we to do to keep our camera steady? There are endless rigs out there to accomplish steadyness, and here we'll talk about gunstock style stabilizers.
So, what's the main thing about gunstocks? Simple: instead of sitting the weight on the shoulder, the weight gets pressed into the shoulder. If you've ever tried a simple regular shoulder rig, you've probably immediately noticed the need for counterbalance over or behind your shoulder to get the center of gravity on your shoulder. The gunstock is a different mentality, the natural thing your arms do is pull towards your body, and the gunstock takes advantage of that. Yes you still need to hold up most of the weight of the camera, but at the same time gravity and natural arm tendency help press it into your shoulder for a very solid point of contact.
So you don't get the all day comfort of a properly balanced shoulder rig, but what you do get is significantly improved stability over handheld, and in a smaller, lighter, and more quickly available rig than the shoulder style rigs.
The less expensive rigs will most likely be rated for smaller cameras, but also might have less adjust-ability. More expensive rigs will likely be built for heavier cameras as well as be more customizeable so the position of the camera and pistolgrip are ideal for your body type.