GNU Emacs Lisp Reference Manual
2.3.16 Byte-Code Function Type
The byte compiler produces byte-code function objects.
Internally, a byte-code function object is much like a vector; however,
the evaluator handles this data type specially when it appears as a
function to be called. See section 16. Byte Compilation, for information about
the byte compiler.
The printed representation and read syntax for a byte-code function
object is like that for a vector, with an additional `#' before the
opening `['.
