Describe the difference between A and B. There is one difference, describe it.
Describe a difference between A and B. There may be more differences, describe only one of them.
Describe the differences between A and B. There are several differences, describe them all.
Describe differences between A and B. There are several differences, describe some of them.
Describe difference between A and B. Wrong indeed
As J.R. remarks, there is some room for interpretation in the third sentence. The writer might not expect the answerer to give all the differences, but that depends on the situation.
If I ask a general open question about a subject the student should have studied, I could ask:
What are the differences between a monarchy and a republic?
I would expect you to answer the main distinctive differences about the forms of government, and not, for instance, whether during the 3rd century ad in western South America, societies resembled a monarchy more often than a republic.
However, if I gave you a text to read and I ask you
What are the difference that the author has noticed between the red and the blue phone?
I do expect you to list all the differences that are mentioned in the text.
In general, when you use the definite article (the), you indicate that you want a description of the specific thing or things you are asking about. So not just some random difference, but the difference, the one difference that exists between A and B - or the differences, the whole set of differences, all of them.
When you use the indefinite article (a or zero, no article), you are asking to describe any difference, or differences, that apply to A and B, but not the whole set of differences that might exist.
There is a little but in this:
When someone refers to the difference between A and B, it usually does not imply there is only one difference!
The difference, often with the emphasized in some way, spoken or written, means the most important difference.
Suppose I write an article about two new phones that come out. They have slightly different prices, they have a slightly different performance and the other one lasts half an hour longer on the battery.
But the difference between phone A and B is the screen: A has a 2 inch VGA-resolution screen, whereas B sports a state-of-the-art 42 inch WTFBBQXSVGA-resolution screen.