SQL: Structured Query Language - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 21

SQL built-in functions
SQL language like all the other computer languages comes with
many pre-built functions. These functions can be categorized
into two major groups. One is called “Single Row” functions and
the other one is called “Group” functions. Single Row function
as the name states gets implemented on single records whereas
Group functions get implemented on the multiple records. These
functions will get clarified in your mind once we start discussing
one by one.
Figure 2-6: SQL built-in functions are divided into two categories.