[tor-relays] Difference Between "Number of Routers" And "Number Of Descriptors"

Hi, So upon checking my relay status at torstatus.blutimage.de, I saw Number of Routers In Cache: Number of Descriptors In Cache: I wanted to double check, what is the difference between “Number of Routers” and “Number of Descriptors”? I know Number of Routers means number of relays currently running. Thank you.