Changing a theme question.

While I am relatively new to WordPress, I have manged to figure out my way around the files. However, I am thinking of trying out this new theme I found, and I was wondering what happens when you select a new theme in your Presentation tab and decide you don’t like it or it severely messes up your site. Can you just change right back to the theme you had and all the files, settings, and tinkering you did goes right back to the way it was? I would appreciate some tips here, and my site is http://www.mytwodollars.com if that helps anyone that knows more than I do! Thanks so much