Minecraft renders are where you take your Minecraft save, convert it into a 3D model, and output an image using modelling or rendering software. It sounds confusing, but it's absolutely not. Using a couple of free bits of software, you can do it yourself. We recently used the technique for a special subscriber's cover of PC Gamer UK, the image for which you can see inside.
If you want to just look at some examples, flip through the gallery. If you want to do it yourself, read on. The instructions are long, but it should only take you around 20 minutes and I swear that anyone can do this.