All three of these terms relate to the perception of stimulus. I can't give you original examples based on your experiences, of course, but I can think of a few common ones and you can add your own.
Absolute Threshold. This is the absolute bare minimum of stimuli that a...
- person must be able to perceive, and, generally accepted, they must be able to feel it 50% of the time it is administered. This is person specific as everyone is different. For example, a pin-prick on a guitar-player's fingertip would have to be harder (because of the calluses) to be felt than on someone who doesn't work with their hands. Another example would be how sometimes you feel the tinniest of feelings that comes and goes and you look down to see an ant on your leg. Depending on where the little guy was stepping you may or may not feel his feet.
Difference Threshold. How different do two thing have to be before a person notices they are not the same? A great example of this are all the "spot the difference" books you find at the supermarket. Two pictures that have only a few tiny things changed that you have to figure out. Of course, in this case you'd know that something was different and be looking for it, which lowers the threshold. If you move something in a room and a person who knows the room well comes home, how much change will it take before he/she notices that there is a difference? Again, this threshold would vary from person to person.
Adaptation. People will get used to a stimulus after time. A great example is your car stereo. When you first turn on your car you're probably turning your stereo down, but as you drive you start turning it up. Your ears adapt to the level of the sound and normalize it. Therefore, to perceive something as "loud" we have to continually turn it up. Same with your eyes or sense of smell. Smokers don't tend to notice how they smell because they are used to it. People will learn to ignore the ugly paint color in their office because they are around it so often, and only think about it when an outsider comes in and complains.