What is the difference between D4 and D3 and 2 and 1 on a Honda Accord?

I have a 1997 honda accord dx i think and it has an automatic transmission but it has four choices with D4, D3, 2, and then 1 (in the order it is when pulling the gear shift back). I drive it on D4, but i was wondering what the difference is between the 4 choices and what the advantages are of each one. thanks