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.
I couldn't even play the game before I downloaded this for some reason, but now it works completely! I also love all of the features. It adds much more depth to the game, such as deciding what your character's history was. 10/10 because it deserves it! Keep it up, man!