Both versions of signage follow the same rule – the signifier (the image or word itself) and the signified (the meaning of the word or image). An example of this is a ‘Push’ sign on a door, when you see the word on the door you know that to open this door you must push it. Another example is the ‘no smoking’ signs that consist of a cigarette with a red circle and line through it. You take note of the image, process the meaning and react accordingly by either not smoking or putting out their cigarette.
My blog entry probably doesn’t make much sense and really doesn’t go into as much detail as it could but if I did it would be a very very long blog entry, so if you are really interested in semiotics I suggest you visit http://www.aber.ac.uk/media/Documents/S4B/semiotic.html
I warn you now, it’s a long long read.