User-defined Functions (UDFs) is a function provided by a developer and behave as a built-in function. It gives to the developer the possibility to enhance the function possibilities of an application (generally a query language).
For a database:
SELECT myfunc(r.a, r.b) WHERE yourfunc(r.c, r.d) < 5
SELECT x, concat(r.s) M GROUP BY x
SELECT W FROM tablefunc(a,b)