The types of essays are many. Descriptive essays differ from the very many types of essays insomuch as they are meant to build a thoroughly elaborate and intense experience for those who read it. The good kind of descriptive papers achieve their aims through the use of not facts, figures, statistics or illustrations, but through insightful observation and good expansive descriptions about the subject matter.
First and foremost, you must clarify and identify the precise nature of the subject matter that is to be described. For instance, the purpose of the essay may be to illustrate something about one of the following:
The precise nature of the subject will direct the direction of your description of it.
You may be writing for a journal, a blog, a newspaper, a peer review or for your dissertation or thesis. For each one, the method of writing and the objective of writing will be hugely different. Therefore, you must have a clear understanding about what your are writing and the precise objective that you are looking to get out of this writing exercise.
Well, there is no one precise answer to this one, just like all the other questions about life, the universe and everything in it. The answer is not 42. What you should describe will depend upon what you are writing about.
For instance, if you are writing about a war zone, then you may perhaps describe the bloodshed, the imagery of the children displaced or killed in the location, the faces and expressions of the women, the amputated soldiers et cetera.
If you were describing Lake Como, you most certainly will not be describing such bloodshed, unless of course there was some reason for there to be such gruesome bloodshed at the serene location that is Lake Como.