What is the difference between “What is the difference?” and “What are the differences?”

“What is the difference?” asks for the main distinction.

“What are the differences?” asks for several distinctions.

Examples:

• What is the difference between “I did” and “I have done”?
  This asks for the main grammar difference.

• What are the differences between British English and American English?
  This asks for several differences, such as spelling, vocabulary, and pronunciation.