funktionieren:to be working (as intended) [as in: functioning]

gehen: to be working (as intended) [as in: functioning] [also means: to walk, of course!]

"Funktionieren" is used to make statements about whether a machine or device is working as it should.
"Gehen" can also be used to make statements about whether a machine or device is working as it should, but is more informal.
To ask how a machine or device works, ask "Wie funktioniert das?"
To ask how something is done, ask "Wie geht das?" To ask whether something can be done, or to state that it can, say "Geht das?" or "Das geht" respectively.
"Klappen" is used to say that something you tried to do worked or didn't work.

Der Kopierer funktioniert/geht nicht.	The copier isn't working.
Wie funktioniert das?	How does that [machine or device] work?
Wie geht das?	How do you do that?
--"Geht das?" --"Ja, das geht."	--"Can that be done?" --"Yes, that works."
Das geht so: Erst muß man...	It works like this: First, one has to...
--"Hat das Experiment geklappt?" --"Ja!"	--"Did the experiment work?" --"Yes!"
--"Hast du das Attachment bekommen?" --"Ja, das hat geklappt."	--"Did you get the attachment?" --"Yes, that worked." [Here, "klappen" is used because it's not so much a question of whether the computer is running properly as of whether the attempt to send an attachment (something you tried to do) worked.]