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.
A wonderful project undertaken by wonderful people, that has managed to breathe life into an old classic, and to make it accessible (and playable) to people new to the game.
Such is the quality of the Patch, that it even was included in the GOG release of the game. I love this.