A render is a picture with its background cut out, so that the image is left, and the background is removed. This allows the image to be used for making images, like wallpapers and signatures. Since pictures are worth a thousand words...
Examples of Non-Renders:
As you can see, this image has a background, therefore, it is not a render.
This image is not a render either. Although it may not look like it has a background, if you set your Hidden-Street theme to “Dark” instead of “Light” (you can change it via the dropdown box on the bottom-left of any forum page), you can see it actually has a white background. Images without a background (ergo, transparent backgrounds) are renders. The transparency will automatically be converted to white when the image is saved as .JPG. Renders are commonly saved with the .PNG file extension.
In semi-rare cases like this one, the image has a white background, but it has been saved as .PNG. Why is this? Well, the renderer is to blame. Instead of removing the background and leaving transparency, (s)he has replaced the background with white because (s)he forgot/didn’t know to add transparency to the picture before rendering it.
Examples of Renders:
A good example of a render. This picture has a transparent background, and it has been saved as .PNG. Though the render job is not very well done, it is, nonetheless, still a render.
Another trait renders tend to have are little “icons”, which are the “signatures” of the person who rendered them. They are usually outside of the render, so that the render can still be used.