Elizabeth, there are several posters here that specialize in this topic. Let me just start out.
Dehydrated food generally refers to air dehydrated food. That is normally done with a lot of warm air passing over the food bits. Many of us have our own dehydrators at home. They are particularly good at foods that have a lot of natural water in them, like wet fruits. Generally the dried food can be kept in air-tight bags for quite some time. You might need to simmer some of these a lot to get the original texture back.
Freeze-dried food processing is done commercial commerically since it involves vacuum and cold temperatures. It costs more money, but it does a more thorough job of removing water. If the processed food is kept completely air-sealed, it can keep for a very long time. If sealed in foil and plastic, it might be good for at least a couple of years. If sealed in a steel can, it might be good for 10-20 years. Fat does not freeze-dry, so meat must be very lean to go through this. Sometimes backpackers will buy f.d. food and tear open the bulky foil and plastic package just a day before they depart on their trip. They dump the food into a skimpy plastic bag, because that will be good enough for a short period of time. If kept for a long time that way, it would degrade a bit. It probably does not become harmful, but the nutrients and flavor will degrade first. You can probably eat f.d. food for a long time, but you might want to take a daily supplemental vitamin pill. Also, you might want to rehydrate the f.d. food excessively, like simmer it, to make sure you get a more natural texture that your body can digest.
Foil packed food such as tuna is air-tight. Also, it is packed in water or oil, which tends to keep the nutrients, texture, and flavor good, at least somewhat past the expiration date on the package. The only problem is that it is a little heavy or a lot heavy. It is almost as good as fresh food, IMO.
If I were heading out on a long trip now, I would likely have some of each of these types.