Gidday Brynn,
Firstly, I suggest reading the thread pinned above titled "Sticky: Comprehensive BG1 Mod List ~Updated 8 apr 2004~" as that details all of the currently available mods for BG1, along with links to their websites. Then, if any of your questions remain, post away.
[QUOTE=Brynn]Hello people,
I read several posts mentioning TuTu and different mods that provide special items or characters, but the whole thing is still not very clear for me.

So
#1. Could anyone please explain me what TuTu is, and what mods are available?
#2. Are these all downloadable stuff? [/QUOTE]
See above.
[QUOTE=Brynn]#3. What are the advantages and disadvantages of using these mods? [/QUOTE]
Some extend the game and add new areas. Some modify the base rules of the game, changing it into a completely different style. Whether these are advantages or disadvantages is personal opinion. Some are buggy and may crash your game.
[QUOTE=Brynn]#4. Do I have to install the mod and start the game all over again or am I allowed to install it during the game?[/QUOTE]
Depends on the mod. Read the readme associated with each mod.
[QUOTE=Brynn]#5. If I save a game when using a mod, is it possible to restore it later into the original game?
#6. Same for characters?[/QUOTE]
Generally not, especially if the mod changes base rules, or you are in an area not present in the core game.
[QUOTE=Brynn]#7. How will these mods or TuTu affect my character if imported into BG II?[/QUOTE]
It again depends on the mod itself. I have heard that there are problems when moving a TuTu BG1 PC to BG2 SoA, but I am not sure. Check out the relevant forums for the mods themselves.
Cheers mate,
TGHO