A Few Quick Questions About Tablets — Best Answer?

I want to buy either a Kindle Fire HD 7″ or a Samsung Galaxy Tablet 2. I will be in college soon so I need it for e-books and apps and stuff like that. Can I download any app off the web on these tablets or are they specific. Im new to the working of tablets so was unsure. I wanted to know because I am thinking about purchasing an e-book through Chegg and I dont know if I will be able to download it onto the tablet. I saw somewhere that you can only use GooglePlay or Samsung Store (for Samsung Galaxy tab) to download apps. However, I am still unclear on how this works.
Thanks
Also, can I download an app on an SD card and put it into the tablet and run it from there? Will it work that way? Is there an easier option?
Thanks again!