As Tim mentioned...stacking is taking multiple photos, and copying them then pasting them as new layers in PSP or Photoshop (or many other programs that allow multiple layers). Essentially, it just becomes one photo on top of another on top of another, and so on. You will only be seeing the topmost photo in the stack as you add more, and usually you'll have a control box or list that will show each layer's name and give you a few options to change how they are displayed. You have the option to change the level of opacity of each or all layers - making them partially see-through - and you can also change how the layers are blended together (screening, which combines the lightness values of the layer photo with the original to brighten it, multiplying which combines both lightness and darkness values to increase contrast and saturation, burning, dodging, color blend, saturation blend, and many more).
If you use two totally different photos, and blend them together in normal blending mode with 50% opacity, you'll get, say, a photo of a person at a beach ghosted over a photo of the castle. It'll just look like two photos accidentally exposed together. But if you take say 5 photos of the exact same scene, with the camera on a tripod, then only things that are moving will be different in each frame. If people are walking through the shot, and you take 5 shots, then stack these 5 shots and adjust the opacity of each layer to 50%, then all of the fixed objects like buildings and light posts will look identical, but objects that are only in 1 place in 1 photo at any time will be partially invisible, or ghosted.
As Tim mentioned, I also like to take slow shutter daytime exposures which also achieve a ghosting effect, though all in-camera. By closing the aperture on your camera, and with the help of a very dark filter, you can leave the shutter open in broad daylight for 5, 10, or 15 seconds or longer. The result will be a scene exposed roughly the same as a normal snapshot exposure - sky, buildings, and other fixed objects will all look normal - but anything in motion will just be a blur or a streak. It works the same as a nighttime long shutter exposure does - streaky taillights, ghostly streaks of people, etc...except you are forcing it to happen in daylight by using very dark filters to fool the camera into thinking it's night.
Anyway...back to the Halloween Main Street thread!