I find that I have neglected giving a full explanation of how to save your files, and what file types you should be using. This will be a short lesson, and some of it may seem very elementary to you. But I wanted to cover my bases on this one. Sometimes I assume people understand something, but it's just a piece that's missing in their knowledge base.
First of all, you need to understand different file types. When you save a file in a certain program, that program has it's DEFAULT setting--what it will automatically save it as if you don't change it. For instance, if you save a file in Microsoft Word, it will be saved as a .doc file. (or .docx if you have Office 2007.) If you save an Excel file, it's a .xls file. In Photoshop, it will save as a .psd file. Usually, these files can ONLY be opened in the program that you created it in. You can't open a .psd Photoshop in Word, for example.
PSD - In Photoshop, as long as the file is a .psd file, you can go back and change it. Your layers will stay there. When you open the file, you can see all your layers and you can delete and add to your heart's content. BUT--.psd files are HUGE. They take up a lot of space. Plus, if you are uploading it to a website for printing, they don't accept .psd files.
JPG - A JPEG is another file type. You can see it on the pull down menu shown above. When you save your file as a JPEG, you are basically flattening the image. When you open it again in Photoshop, you will not be able to see your layers. It's now one flat picture and you can't make changes. It's also a smaller file. You want to save your files as JPG's so you can upload them.
DON'T DELETE YOUR PSD FILES!!! When I finish creating a page, I save it as a .psd file. Then I save it AGAIN, usually in another folder, as a .jpg. I keep both copies. Often I'll upload my .jpg for printing, and then notice a spelling error or maybe my margins are too narrow and my text is going to fall off the page. I might need to go back to my original .psd file, make some changes, then resave as a .psd and a .jpg and reload. That's why I don't delete my .psd's.