WeiDU is the program used to install mods on your game, in a way that any changes the mod makes can be taken off again (kind of like layers on an onion). This means that any time you decide to change your game, you can change the mods around without reinstalling the whole game from scratch. Plus, if you decide you don't like a mod, you can take it out. The good news is that WeiDU mods are distributed with a SETUP-NameOfMod.EXE, which is WeiDU (Weimer Dialogue Utility, after the dude who wroye the program originally).
You definitely do *not* want to install a non-WeiDU mod if a WeiDU verison exists. Trust me. The non-WeiDU mods are older technology, which replace things by overwriting them. Patch = good, Overwrite (in general terms) = bad.
"Tutu" is "Two Two" > "BG1 To BG2". Basically, it is a huge Mod of BG2, which lets you play BG1 in the BG2 engine. It's latest incarnation is EasyTutu, which uses an InstallShield program, and makes a special directory and standalone game of Tutu, so you can play BG1 and BG2 side by side as different games.
A similar technology with a very different implementation is "BGT" - "Baldur's Gate Trilogy". BGT is a WeiDU mod that ads a transition scene and allows you to play the whole saga from CandleKeep L0 to Godhood in one install. It is often associated with folks who like "BP", or "Big Picture" installs, where modders have looped very large mods (101MB+ compressed) into a single game and made them more or less work together. BGT actually can be played as an independent game, without BP or that whole famoily of mods.
A good start to make decisions on this would be to look around some forums and see what people like and don't like. Here, and at G3, folks tend to be Tutu players and keep their BG1 and BG2 experiences separate. I'm one of these - a game is long enough for me that I want the freedom of changing my BG1 game with one set of mods, and my BG2 game with another, and I even want to have a character run in both. Over at SHS, BGT is the standard - folks there like the idea of using many big mods together to change the game experience, and keep everything in one long, seamless game.
If you are new to adding mods (as you indicated), I would say the best experience for you would be to chose a *few*, small number of mods, and read all about them on thier forums. Try playing a game with one or two, and see what it is like to install them, change them, and tweak your game the way you would like. then you can have some fun and go check out the whole gamut - everything from mods taht improve the AI, to ones that add new companions, to ones that actually recreate a whole new game using the basic engine. Lots out there - but wade on in. The water is warm, there are plenty of lifeguards around, and worst comes to worst - it is all just 1's and 0's. Worst case scenario, you uninstall and reinstall