Noise is the presence of unwanted dots of color randomly distributed around a photo, resulting in a speckling effect. Due to its random nature, it is difficult to remove noise. However, it is possible to reduce its impact in a photo.
See Wikipedia for more information on noise.
Using the NL Filter
The NL Filter will typically remove most noise, but reduces the sharpness of the photo at the same time. See below for an example.
Open a photo in the GIMP Image Editor
Press Filters -> Enhance -> NL Filter...
Set the Filter to Optimal estimation
Change the Alpha to a moderate value, around 0.50
Change the Radius to a moderate value, around 0.50 to 0.70
- Tweak the settings to get the best results as seen in your preview
Press OK to apply the filter
You may want to repeat this procedure a couple of times if you have a very noisy photo.
See the GIMP Documentation for more information.
The photo on the left is the noisy original (the noise was actually added artificially for demonstration purposes). The photo on the right is the result after applying the NL Filter using the instructions above.