The algorithms vary (z-buffer, ray-trace, etc.) but generally render means to create a picture composed of discrete pixels. Each pixel has a color
associated with it and the array is what you see when you look at a computer monitor.
Rendering is the process of generating an image from a model by means of computer programs. The model is a description of three-dimensional objects in a strictly defined language or data structure. It would contain geometry, viewpoint, texture, lighting, and shading information. The image is a digital image or raster graphics image. The term may be by analogy with an "artist's rendering" of a scene. 'Rendering' is also used to describe the process of calculating effects in a video editing file to produce final video output.
http://en.wikipedia.org/wiki/Rendering_(computer_graphics)