The Unofficial Patch is a combination of the default Basic Patch which mostly fixes bugs only, and the optional Plus Patch, which is kind of a mod adding various tweaks and implementing a lot of unused content, including new quests, levels, weapons, items, characters, dialogue, graphics, models, sounds and original music! Included are also the optional Patch Extras which feature Player Mods, like shaders suites, as well as Developer Tools, like the Unofficial SDK to mod the game.
Really made the game playable, though now there are better alternatives. I would recommend the Camarilla edition, which adds much needed replayability value to the base game without tinkering with it too much and destroying the atmosphere. A great balance I would say.
If for some reason you don't like that mod, then the unofficial patch solo is for you.