In the market of my galaxy tab my friend signed in using her account and now I am trying to use it but I cant, so how do I signed her out and sign in using another account

I’m not sure exactly if I worded the question right.. but here is my scenario:
I got the iphone4 in October 2010. In February of this year, I got a MacBook Pro 13inch. For christmas I plan on getting an IPad 2. I would just like to know now what an IPad can do that the other two cannot, and/or how it is useful in ways the other two are not. Thanks!

Can you buy and sell on the stock market with a Samsung Galaxy Tab? If not is there a tab that i can do this with. Thanks much, much love.

I recently bought a book over wi-fi and I was reading it this morning and then again in the afternoon, but now I can’t access it anymore. I think it must be saved on cloud storage, but I don’t have wi-fi at my house so I can’t get to it. How can I change this? Oh and also, is there any way to connect my Kindle to the iternet without wi-fi? Like if I connected it to my computer, would that work?

Well, i’ve been trying to transfer this file to my galaxy tab from my laptop, can you guys help me, please? Also, How can i access "My Computer" in Galaxy Tab?