ENBSeries is 3d graphic modification for games like TES Skyrim, TES Oblivion, GTA, Deus Ex, Fallout, and many others. It work by modifying render functions calls of the games and applying additional effects.
It's divided to generic versions and exclusive. First category allow to run it on thousands of games, not all effects will work, but you can try several versions of mod to find best suited. Second, exclusive types of the mod, are games specific versions which developed to work with some game and rarely with same game engine.
Available effects are SSAO, SSIL, Depth Of Field, Lens FX, Bloom, HDR, Tone Mapping, Sharpening, Vignette, Sun Rays, Shadows, Detailed Shadows, Reflection and others in various versions of them.
ENBoost is patch from ENBSeries which reduce memory usage of the games, so you may install much more modifications without crashes and errors, also it better utilize video memory of modern videocard.