Function

body & or Function[body] is a pure (or "anonymous") function. The formal parameters are # (or #1), #2, etc.

x |-> body or Function[x, body] is a pure function with a single formal parameter x.

{x1, x2, ...} |-> body or Function[{x1, x2, ...}, body] is a pure function with a list of formal parameters.

Examples

Create a pure function using &:

(#^2 &) /@ {1, 2, 3, 4}
(* {1, 4, 9, 16} *)

Using named parameters:

Function[{x, y}, x + y][3, 4]
(* 7 *)

Please visit the official Wolfram Language Reference for more details.