World of Warcraft Beginner help - Easy 10 pts.?

Well i'm thinking of buying WoW and I am completely new to it, my 10 day free trial thing doesnt work but i dont care about that. I just have some questions. What are the expansion packs about? Are they like DLCs to consoles? Just an extra world to explore? Are they necessary, or atm, for me will the original disc suffice? Can mac and pc players interact in the same world or are the seperate? What is the main aim of WoW? Can people PK anywhere? (so is it unfair for new players because i have heard that newbies get crushed by higher levels and theres nothing they can do, even if they just want to do a quest on their own). ?? And what is the main charm of the game? And who are the main YouTubers for WoW, like commentary or video-wise. Even if you can only answer some of the questions - all is welcome. Sorry for all the questions but i would be very grateful for an answer :)).