The three forms eval, apply, and funcall allow efficient calling of functions, depending on the form of existing data.
Use eval when the existing data is already Lisp code:
(eval (subst 10.0 'r
'(* 3.1415926 (expt r 2))))
Use apply when the existing data is already a list of arguments:
(setq nums '(3 7 21 14)) (apply #'+ nums)
Use funcall when the existing data is in the form of individual arguments:
(setq x 7) (setq y 3) (funcall #'+ x y)