or, Dots, Pixels, and Inches.
Ok, this can be confusing. I'm going to try to explain it the best I can understand and cover what you need to know for digital scrapbooking. I'm not going to go in-depth because it gives me a headache.
Have you ever heard (read) the terms "DPI" and "PPI" and wondered what in the world they are? DPI is DOTS per inch. This is technically a term for PRINTING. The higher a printer's DPI capability, the high quality image the printer can produce. Now, our digital images are made up of PIXELS. Thus PPI -- pixels per inch -- is technically the term we should be using when talking about a picture's (or layout's) resolution. But somehow along the way, the terms DPI and PPI became interchangeable, so you will hear either term used to refer to digital image quality. Within Photoshop, next to the Resolution box, it says "pixels/inch." In "Image > Image Size," the top part you can deal directly with pixel dimensions, and underneath that is document size in both inches and resolution.
Here is a definition I found in a Photoshop book:
The number of pixels in an image = the resolution x the image dimensions.
Thus an image at 100 ppi at 8x10 inches will be 800 pixels wide and 1000 inches tall. (That's why I like 100 ppi, the pixels are easy to predict.)
An 8x10 image at 300 ppi would be 2400 pixels wide and 3000 tall. On your desktop or exploring folders, you can see these dimensions for each picture when you have the area active where the pictures are and you hover the mouse over each picture. The pop-up box would say for the above 8x10 at 100 ppi example, "dimensions: 800 x 1000." Next it will say what kind of an image it is (JPEG) and how much file space the saved image takes up in MB's or KB's.
If an image is printed larger than there are sufficient pixels, it will look pixelated. It won't be a smooth, clear image, you'll be able to see the pixels the image is made up of (it will look like a bunch of little boxes.) Because of this, when you are creating something with the intention of printing it, you need to save it at the right resolution. Now, there is some debate about 200 ppi versus 300 ppi in terms of Photoshop speed, but I personally like to have my pages and pictures all at 300 ppi. So when you're starting a new workspace for your scrapbook page, make sure the resolution is high enough. It's easy to size down, but next to impossible to size up and retain quality.
Saving for the internet is another matter. I don't remember/understand the technical reason why, but for the internet high resolutions don't make much of a difference for viewing. The recommended ppi to save for internet usage is 72. I like using 100, but that's just me. So long story short: for most people, use 300 ppi resolution for all pictures and layouts, and use 72 ppi resolution for sharing your stuff on the internet. But be VERY CAREFUL after re-sizing to a smaller ppi that you not save over your image like this. Once you save over a file and then close it, you can't get those pixels back, and that's SO not cool! I'll talk more about the saving process here.