Unofficial mods are mods that people have put together using some tools that allow them to edit the Bioware Infinity Engine (which the BG and IWD series runs on). Examples would be any of the editing tools that you can get from
www.teambg.com, such as Infinity Engine Editor Pro, Script Editor, etc. There are also other tools like Near Infinity, Infinity Campaign Maker...the list goes on.
At its most basic, a mod might be a customized, personal weapon that you create for yourself, with some specific ability. You may then decide to create some additional dialogue with some existing NPCs in the game. You may then extend this to a romance dialog eg the Solaufein mod pack or the Nalia romance. At the pinnacle of modding would be in creating whole new modules/maps/encounters/storylines eg. like the Darkest Day mod.
The quality of the mod is obviously totally dependent on the ability of the people putting them together. The more complex the mod, the bigger the chance that there will be bugs. Usually the bugs are limited to the mod either failing to work or working in an erratic way. Sometimes it may cause another part of your game to not work. Sometimes it might work perfectly fine. [img]smile.gif[/img]
It's unusual for a mod to corrupt something else in your hard drive - but I can't guarantee this, since something that works for someone might not work for someone else.
Forums such as this are a good idea to get feedback from people on which mods they have tried and which ones work well and reliably. One thing I will say though is that the amount of reverse engineering (modding) that people have been able to do in the last year or so has been quite amazing and they're getting better all the time. [img]smile.gif[/img]