GIMP makes a distinction between saving and exporting images.
Saving an Image. The image is stored with all its properties in a lossless format. This includes, for example, layer and path information. This means that repeatedly opening and saving the image will neither degrade its quality nor how well it can be edited.
To save an image, use File › Save or File › Save as. To be able to store all properties, only the native format of GIMP is allowed in this mode: the XCF format.
Exporting an image. The image is stored in a format in which some properties can be lost. For example, most image formats do not support layers. When exporting, GIMP will often tell you which properties will be lost and ask you to decide how to proceed.
To export an image, use File › Overwrite or File › Export As. Below is a selection of the most common file formats that GIMP can export to:
JPEG
A common format for photographs and Web page graphics without transparency. Its compression method enables reduction of file sizes, but information is lost when compressing. It may be a good idea to use the preview option when adjusting the compression level. Levels of 85% to 75% often result in an acceptable image quality with reasonable compression. Repeatedly opening a JPEG and then saving can quickly result in poor image quality.
GIF
Although very popular in the past for graphics with transparency, GIF is less often used now. GIF is also used for animated images. The format can only save indexed images. See Section 17.6.7, “Image Modes” for information about indexed images. The file size can often be quite small if only a few colors are used.
PNG
With its support for transparency, lossless compression, and good browser support, PNG is the preferred format for Web graphics with transparency. An added advantage is that PNG offers partial transparency, which is not offered by GIF. This enables smoother transitions from colored areas to transparent areas (antialiasing). It also supports the full RGB color space which makes it usable for photos. However, it cannot be used for animations.