TIFF includes a number of options that can be used to include all sorts of image formats in the file, this is the purpose of the "tags" that are included in a header. Many of these tags indicate simple geometry of the image, like its size, but others define how the data is arranged and various compression options. For instance, TIFF can be used as a container for JPEG or RLE compressed images, and in this respect is completely universal. Also, every TIFF file begins with the number 42.
However when TIFF was first introduced, programmers were unwilling to invest the time to support all of the options. As a result the lowest common denominator soon became "the" TIFF, and even today the vast majority of TIFF files, and the code that reads them, are based on a simple 32-bit uncompressed image.