updated November 27th, 2016
ENBSeries is a graphical modification for many games, including Skyrim. It uses a modified d3d9.dll file, implementing new graphical options such as ambient occlusion, indirect lighting, Bokeh depth-of-field, lens effects, better bloom, HDR, tone maping, parallax maping, vignette, sun rays, detailed shadows, film grain, reflections and much more to come in later versions.
The settings can be modified and adjusted to your liking. Many people have released their own ENB presets which they use and wish to share.
And no, the creator won't tell us what the ENB name actually means. I'm serious, he won't. Don't attempt to figure out what it stands for and just remember what it does; it makes your games look pretty. Go to the official website for more information.