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.
The game can be really frustrating at times due to occasional game-breaking bugs. That's why I highly appreciate the extreme amount of effort that is put into these patches. I can't really thank the creator enough for these incredible unofficial patches. Thank you so much!!!