A question on Addons, from the beginning please!

[possumboy]

I have been playing WoW for a little over a month now, and I have a question about how the Addons work.

I came to WoW from EQ, and although I was by no means an expert on UI's there, I felt comfortable with switching whole UI's/components. With WoW, things seem significantly different.

For example, I am having trouble getting my head around whether Cosmos (for instance) is a 'program' that addons plug into, and whether Cosmos is a separate program, to say, Ace2.

To try and explain what I'm getting at; I have been using Cosmos with Auctioneer, Gatherer and MapNotes among others, as recommended by RL friends that play WoW. Tonight I tried to DL and use something called TradeSkillInfo, which looked fab! I couldnt get that one to work, kept throwing up errors.

I am unsure as to what all the dependancies are, and whether the probs are linked. When I first setup Cosmos, I just downloaded a job lot of things, under the assumption that if I try to self teach myself, I wont have to bug people here as much! Heh, worked with EQ!
I understand that the dependancies are required for certain addons (they are stated in the addon window), what I dont understand is how interchangeable addons are (if indeed there are addons written for specific progs, IE Cosmos OR Ace2 etc)

I guess one of the questions I am asking is will addons cause conflicts with each other?

As you can see, I have gotten my head into quite a spin over these questions, and I wouldnt be surprised if you want me to clarify a few points! I have tried looking for info on these issues, both in these forums and elsewhere on the net, but I couldnt find the basic info, only what things do in-game and where to get them from.

ANY help would be great really! I always think that you cant understand just bits of info, you need to work from the ground up. Now I just need the facts to build some foundations!

Thanks for reading