In Save As dialog box you can choose one of 16 graphic formats to save your image in. However most of us only use 4 or 5 formats.

1. PSD (Photoshop Document) is the working (intermediate) format. It saves full information with your work (layers, masks, and text). We use it to be able to edit the file later should we need to.

2. GIF format is rather good for Web graphics - buttons, banners, and text. The fewer colors you need, the smaller the file size. The best things about the GIF format are that it supports transparency and animation (both features are important for Web).   The GIF file format is generally not a good choice for full color photographic images, because of its reduced color palette.

3. JPEG is a popular format, most suitable for publishing your photos on the Web, sending via email and is quite good for printing (though some people think different). When you save an image in JPEG format you can optimize the  "image quality / file size" ratio in the JPEG Options dialog box . The bad thing about JPEG - it looses image information. The smaller your file size (lower quality setting), the worse the image looks. 

4. TIF format is highly respected by imaging professionals. In fact it's a standard output format for quality printing. It doesn't loose image info, but produce very large files. 

5. BMP format is widely used in the Windows environment. That's probably why it's good for saving quality screenshots. Photographic image quality and file size are about the same as with TIF the format.