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.bmp Windows bitmapped image. Used by Microsoft Windows applications. Good quality, large file size. Not for use on web.
.wmf Windows MetaFile. Useful for clipart, and can be used to make large area, small sized background files.
.pcx Older general purpose format. Practically obsolete now. Not for use on web.
.psp Internal format for Paint Shop Pro, useful if you want to swap files between these two applications.
.pcd Kodak PhotoCD format, used with Photo Developing - although most photo processors will save your photos as jpegs if you ask them to.
.pdf Portable Document File. Adobe's file system that allows electronic cross platform sharing of documents.
So what's best format for web graphics: gif or jpeg?
The general rule of thumb is to use gifs for diagrams, line drawings, illustrations, and images that contain large areas of flat colour, and jpegs for photographs and images with continuous colour tones. The jpeg format has a very good compression rate, but compression reduces quality of image, so it's best to experiment with tools in Image Ready until you have optimal quality/file size. The gif format, on other hand, has a smaller file size - but a limited range of colours.
Shaun Pearce is a writer and video maker. His latest production, "Photoshop Master", is an interactive video tutorial. It shows you how to get the most from Photoshop, and can be downloaded from http://www.learnphotoshopfast.com?=art03g