According to the SAS Online Documentation, SAS functions are defined as:

“A SAS function performs a computation or system manipulation on arguments and returns a value. Most functions use arguments supplied by the user, but a few obtain their arguments from the operating environment.”

You can see details on what SAS functions are and how they can be used at SAS Functions and Call Routines overview online documentation page.

You can also see a complete list of the functions available at the SAS Functions and Call Routines Dictionary online documentation page.