Yes, spoken chinese is so different from one area to another it can't be understood.. but all the languages use the same characters to mean the same thing.(so any one can read something and come to the same understanding.)
think just of american engish and english english..
"fanny" has very different meanings.
"schedule" (is that skedule? or shedule?) is spoken differently)
to hospital? or to the hospital?
now imagine, the changes that can, and did occur over the thousands of mile that chinese is spoken, over thousands of years!
Mandrin is one main "dialect", Cantonese is an other.. there are thousands.. some are fairly close to each other, but all have some differences..
a character in chineses might represent a period of time (from new moon to full) it could be said as "two weeks" or "for'tnight", or 14 days, all very different words, but each represents the same interval of time.
but if you "said" it as for'tnight, many here in US would not know what you meant.. So too with chinese.. the idea (an interval of time that is 1/2 of a lunar month) can be expressed in many different "sounds". Not every one knows all the different sounds, but if presented as written chinese, it would be understood by all of the "speakers".