In digital photography terminology a TIFF is a type of lossless file format. Stands for Tagged Image File Format. Because they are lossless files the image data is stored whole (nothing is thrown away*). This results in large file sizes, but also good quality photographs.
* TIFFs can be compressed to save space, but not as much as JPEGs. When they are un-compressed the software reverses the original compression process; the original large file is the result.
Glossary of Digital Photography Terminology – http://www.digital-photography-tips.net