GIMP's files are saved in XCF format while you're working on them, but once you have a completed image, you need to save to a different file format, depending on the type of image and how you intend to use it.
TIFFs are a popular format with photographers because, while they offer the option of compression to reduce file size, TIFFs can be saved with no compression for maximum quality.
1. Save the Image
© Ian Pullen
2. Export File Dialog
© Ian Pullen
TIFFs cannot contain the same amount of information as an XCF file, such as layers, so if the Export File dialog appears, you will need to click the Export button. It is generally best to accept any default settings at this stage.
3. Save as TIFF Dialog
© Ian Pullen
You do have the option of saving a TIFF using one of several different compression types, however, for most people, the attraction of TIFFs is that they maintain maximum image quality without compression. Most users can ignore Save color values from transparent pixels, though adding a comment to an image maybe useful for some.
4. Conclusion
While GIMP doesn't offer quite the same degree of flexibility in saving TIFFs as Adobe Photoshop (which can save layers in TIFFs), it does offer several compression options--though for many users the great attraction of TIFFs is using them without any form of file compression.
If you want to save an image from GIMP that you can later work on many more times, TIFFs are ideal as you won't be losing any image information each time you save your image if you choose not to use compression.