My dear Gillian, thanks for always replying to my question about which ones you like more!
About the long exposure, basically means you can take photos without light with a low ISO. ISO is one of the 3 main parameters you have to deal with to take a shot. The other 2 are shutter speed and aperture. They depend of and interact with each other. And through their combination you can take the photos you want to, with the effects you wish to.
So, without light, like in night photography or in the dark, if you want to take a photo and have a tripod, you can lower the ISO and get a photo with no noise (grain). If you don't have a tripod, you need to higher the ISO to take a photo, and then you get one, but with noise (after you can edit them and reduce it, but the result is never the same - unless you want THAT result!).
With a long exposure photo, meaning the shutter speed as low as you need (most of DSLR cameras have 30 sec. by default, but you can choose BULB setting and then choose the time you want the exposure to be), depending on the aperture (for landscapes a smaller one is desirable, so you can have all the image "focused"), you got this:

• every still subject, as a rock, or a house, or a tree (if there is no wind), meaning things that don't move, will keep still and "focused";
• every moving subject, as people, or car lights, or sea waves, will have the "trail" or the "silky" effect, depending of how long the (time) exposure is.
  There are filters that allow you to take long exposure photos during the day also, that's why you can see those beautiful silky waterfalls long exposure photos on the daylight.
  And long exposure allows you to play with endless situations: for instance, in a crowded place at night, like a famous square, if you are lucky with the people moving, the result will be the space to be empty of people.
  And get light trails of light moving, like my first photo, or the star trails.
  If you have a phone with "live" setting for photos (I do have one, the Iphone X), you can simulate a long exposure with a life photo (by pushing the photo up and choosing the setting long exposure).
  I hope I have been clear, I tried to keep the explanation simple, but if you have any more questions just "shoot" them back!