Well actually, there is only one but there are a few different ways that the word friend can apply. You can have a friend that you really only spend a little bit of time with and never really get to know. Then you can have a friend that is super strong with you and that you share all of your secrets with and spend all of your time with. Then you can have an adult friend that you know very well but only spend time with at work and the occasional social gathering. Then you can have childhood friends that you are super close with and that you see nearly every single day.
First of all to find out the true significance of a dream like this you are going to need to find out how close you are to your friend and the only way to do that is to be totally honest with yourself. If you dream of a dead friend, the death of friend, dying friend, or a dead frog, you need to know how close you are to that friend or frog before you attempt to interpret since the results in interpretation will vary so greatly. Do this and then move on.
If the friend that was dead in your dream was one that you were very close to in life then to see those dead in your dream means that you feel like your relationship is starting to split apart. There are many reasons why fiery friendships die or grow cold. If you are very distant emotionally or physically this is one reason why friendships do not go so well. If you move on to college or move on to a job in a new city this might make it hard to see your friends. When you are outside of the distance between their house and yours by a great deal this will make it harder for them to see you.
Either way, seeing this dead friend in your dreams means that you want to see them and that you are mournful of the fact that your relationship seems to be dying. You want your friend to come back to you and you want to see them again to make sure that they turn back to you and are still your friend again. The best way to go about this situation is to try your hardest to go and see them again where they are. You need to get together with them again lest they start to feel a lot more disconnected from you. The only way to get closer again is to spend more time together and the only way to do that is to be physically closer together.