Stacking or tracking
Several of my astrophotography pictures are made by stacking photos. The night sky is much darker than cameras are typically designed for. To compensate for this, I take long exposures (leaving the camera open longer to collect more light). However, the earth is rotating! So it appears that the stars are moving. This means that too long of an exposure causes the stars to appear as lines crossing the sky (star trails). There are two ways to compensate for this. One is stacking. This involves taking several exposures which are long, but not long enough for star trails. Those exposures are then combined afterwards on the computer. Doing this makes the stars clearer, insures that as many stars as possible are included, and reduces any weirdness (noise) introduced by ambient heat or random electrical activity on the camera sensor. The software compensates for the fact that the stars have moved a little bit between each picture. If I take 20 pictures and each one was a 30 second exposure, the stacking software tries to make this equivalent to taking a single, 10 minute exposure. Tracking involves mounting the camera on a special motor so it rotates with (tracks) the stars. This allows for a much longer exposure without star trails, and can eliminate the need for stacking afterward. Because both of these focus on the stars and compensate for their movement, they produce pictures with very clear stars and very blurry foregrounds! Which leads me to...
In astrophotography, I can get a clearer, brighter picture of the foreground (for example, Mt. Rainier) by taking a longer exposure with lower ISO settings. This is relatively easy, because the foreground doesn't move (unless it's a tree on a windy day, or water, etc.). But the background will now have star trails and the Milky Way will be blurry! So now I have stacked or tracked and gotten clear stars and blurry foregrounds, and I have taken a long exposure and gotten a clear foreground with blurry stars. So next I use masking to combine the clear foreground with the clear stars. Masking is simply a photoshop technique which allows you to keep the parts you want from multiple images after layering them on top of each other. For more information on this, there are many tutorials available on the internet.
Some of my landscape prints use long exposure. I typically accomplish this by putting the camera on a tripod, using a cable release or intervalometer, and using neutral density filters. A good example would be the pictures in my Snoqualmie River gallery. These pictures were taken using exposures which ranged from 12 to 30 seconds. I composed the shot, took a test picture, and then put the filter on. The filter is called a "neutral density" filter because it filters out all colors evenly. Using a neutral density filter allows me to convey movement (the ripples in the water are either smoothed out or captured as white streaks). Why did I take a test picture first? Because with the filter on, I can't see through the camera! It's too dark. The exposure has to be several seconds long, just to let in enough light!