I have the DuWop and Cargo Reverse Liners. I tend to favor the Cargo Reverse liner use it pretty frequently.
The Cargo Reverse liner has a light, neutral shade to it. I realize that neutral is a relative term, so to put it in perspective, I am an NW25. It is the size of a regular lip liner pencil. It is a slightly waxy texture. You use it to outline the lips. It gives them a nice even shape and helps to hold in lip product like lipstick or lip glass. As moonrevel said, it really helps to eliminate "feathering". It almost highlights the area around the lips, making the lips appear fuller, with a cleaner line. I put this on after my foundation and blend it in with a synthetic brush, so that it looks natural (I use a #242 MAC eyeshadow brush).
The DuWop Reverse liner, which is a bigger "chubby" style pencil that works much the same way, except that it is a transparent wax. Still needs blending in though. Because it is transparent, you loose the ability to shape/highlight the outer lip area. Some people, and myself on occasion, feel that the Reverse Liners (Cargo and DuWop) are too emollient, so I set them with a powder when needed.
The one bummer about the Cargo shade is that if someone has quite a dark skin tone, it doesn't work all that well. Which brings me to.....
Another way to accomplish the same thing as the Cargo Reverse liner is to just use a concealer and powder. You can line around the lips with concealer (you can use your foundation shade or go a shade or two lighter than your foundation), blend it in and set with powder. The MAC concealer pencils work well, or use the MAC Studio Finish concealer (the one in the pot) with a #263 brush.
The Cargo tends to work better than the concealer trick, as its texture is more waxy and less emollient than concealers, but concealers are a close second that works just fine for some.
And believe it or not, I am now done yammering on about reverse liners.