Tiff is an image format and the recommended one for use in DiTRL . Tiff is somewhat uncommon to the average computer user since it is never used for web graphics. However once people start working with other media such as posters, slides, etc. Tiffs become very common. Compression is a way to reduce file size without decreasing the size of the image in pixels.
Compression usually involves the loss of colour information or pixel information.
When pixels are lost, the compression scheme selects certain areas and replaces them with fewer kinds of pixels. This kind of compression is used in JPEGs, GIFs, etc. Losless compression is a way to pack data into a smaller space without data loss. Consider it similar to the way chromatin is condensed by histones in cell nuclei. The same amount of information is there but the data is not readable.