GIMP stands for GNU Image Manipulation Program. This programs only works with images. You can use this program to:
All files end in a "file extension", usually a 3 or 4 letter extension. For example GIMP's native file ends in a 3 letter ".xcf" file extension. You would save your images using the .xcf extension when using advanced techniques like layers. Otherwise GIMP will save the image in the same format that they are opened in. For example, if you opened a .jpg image and modified it and saved it, it would still be a .jpg file.
Other than GIMP's native .xcf file, there shouldn't be too many compatibility issues when opening/saving industry standard image file types, such as .gif, .png, .tif and .jpg images.
The GIMP topics are shown on left under the heading "GIMP Topics". Choose your topic, you will always know which topic page you are reading because the topic link will be white indicating you are on that topic.