What is the difference between an explain and describe question?

If a question asks you to explain it wants you to say WHY something happens. Whereas if the question says describe it wants you to say WHAT happens. 

For example; DESCRIBE the short term effects of exercise on the heart?

We get an increase in HR .

EXPLAIN the short term effects of exercise on the heart?

HR increases due to more oxygen being needed at the working muscle, so the heart beets faster to deliver more oxygenated blood to the working muscle. 

Another great example would be using a graph or table.