Joke aside, the Codec Pack is a Windows operating system -level add-in that enables Windows to read many new image formats (44 file extensions at last count). Windows Explorer gets the ability to create thumbnails from images formats it does not normally understand and, on Windows Vista and Windows 7, the built-in image viewers (Windows Photo Gallery and Windows Photo Viewer, respectively) also get the ability to open new image formats, as well as Windows Media Center on Windows 7 and several 3rd-party applications.
Our codecs makes great
thumbnails on XP SP3 too (better than other existing codecs, for example they remove the black stripe that sometimes appears on the borders of some thumbnails, depending on the camera that created them, and perform high-quality reductions, as illustrated below with our DNG codec (top):
All details are on the Codec Pack web page
FastPictureViewer Professional, on the other hand, is a stand-alone image viewer specifically created for fast image culling and rating. FastPictureViewer Professional supports the same raw formats as those enabled by the Codec Pack (same camera models etc), while the Codec Pack adds about 10 extra formats on its own, including Photoshop PSD, as well as a special JPEG codec that works on Vista and Window 7 and performs EXIF-based automatic rotation of JPEG images.
Depending on your needs, you may want to install one or the other, or both. The two products are designed to work well together and complement each other nicely and FPV Pro gets access to more images formats with the Codec Pack installed.