Every year, you can waste valuable time in exams by explaining something, when the COMMAND WORD simply asked you to describe something. Know the difference!
DESCRIBE: Examiners are asking you to state what you can see (on a photo, a diagram, or a graph etc) or simply say what something is like. In some subjects it may be asking you to give an account of events.
EXPLAIN: These questions are harder because you will need to show your understanding by giving reasons for why or how something happens.