what is difference between : and :#

I was curious about the difference if any between something like

(defpackage :dummy
(:use :cl))

and

(defpackage #:dummy
(:use #:cl))

which is preferred?

:word creates a symbol in the keyword package; it will remain during the life of your program.
#:word creates a symbol in no package; it will presumably be garbage collected after use.

In both cases, symbol-name is used to create a new symbol (by name). The benefit of using these instead of "word" is that they undergo the default case translation whereas a string literal does not.

Thus there is a minor memory benefit to using #:word.

nice explanations thx