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.
This mod polishes a gem that Troika has created. Not only that, it also adds new levels and content that was cut due to time and budget restrictions. The amount of effort Wesp5 has put in deserves highest praise. Hats off to you :).