Need to know: Each camera has a maximum number of pixels (dots) that it can use to make up a picture. The more pixels, the better the resolution (smooth looking picture with good detail.) Fewer pixels means a grainy, blurry picture with poor detail (grainy means being able to see the individual pixels instead of just nice smooth skin.)
Optical zoom is the same as walking closer to your subject without actually having to do so. The subject gets bigger, and this also increases your resolution. So, for example, a face goes from being made up of 1,000 pixels (little dots) to 2,000, so your picture will not be grainy.
When you use an optical zoom, it does NOT increase the resolution. It is the same a blowing up a picture on your computer screen to make it bigger; the face gets bigger, but the number of pixels stays the same, so the picture gets grainy looking.
Bottom line: You want to minimize the use of the optical zoom as much as possible because you get a grainier picture.
Another note on optical zoom claims. I've noticed that they often are rating the zoom (for example, 10X) based on a lens that starts from a wide angle view. So you are not going to be able to zoom in on a distant subject as much as you are expecting from a normal (actual size) view. You will use up maybe 2X or 3X of your zoom just to get to a normal view from the baseline wide angle view. Which means you really are only getting a 7X zoom to get closer to a distant subject. If you're into something like photographing wildlife or landscapes, that can make a big difference. Of course, you can blow it up and crop it later, but you will be losing resolution and getting a grainier picture. Not all lenses start with wide angle as the baseline, but a lot of the cheaper ones do. So check it before you by it by actually looking through the lens.
Hope this helps!