You need to arrange 7 of your favorite books along a small shelf. How many different ways can your arrange the books? Assuming that the order of the books makes a difference to you.
You have 7 books for the first slot, 6 for the second, 5 for the third, etc etc until you get down to 1 for the last slot. Multiply all these choices together to get your final answer.
This is the same as 7 factorial (7!)