Page Formatting Question

I am new to Word Press so I apologize if this turns out to be a dumb question.

I am building a membership site with WordPress, using WPWishlist, but I wanted to ask if the way the content on each page is laid out is based on the chosen theme I am using?

I am trying to figure out how to add new content each month for the pages in my site (not posts) in an organized and easy manner.

Can someone point me in the right direction?

Thanks in advance.