Most people find the iPhoto and iTunes applications an easy way to view organize pictures and music, and iPhoto has great slideshow, printing and editing features as well. One drawback of both programs, however, is that they "hide" your files from you, making them difficult to find or manage on your own. Because both programs use a database to help organize your files there can also be times when the program "loses" your files, making you think all is lost.
So where ARE your picture and music files? iPhoto stores everything in the iPhoto library, located in the Home --> Pictures folder. Although this looks like a single file, ctrl-clicking on it and choosing "show package contents" will show you a folder that includes many database files and a folder called "originals" that has your actual JPEG images. This understanding can be particularly helpful if your database becomes corrupted: you can pull out the "originals" folder, rename or delete the iPhoto Library and re-import them.
iTunes works in a very similar way, however the "iTunes Music" folder containing all of your actual files is not hidden: it is located in Home --> Music --> iTunes.