It's like difference between american english and british english. they can communicate each other with english but some words and accents are different.

It is like differences between mainland China(Mandarin) and Taiwan(Taiwanese) . Although South Korean and North Korean can communicate each other practically, there are some differences in pronunciation, orthography and vocabulary. For detailed information, refer to http://en.wikipedia.org/wiki/North%E2%80%93South_differences_in_the_Korean_language

Richard, thank you for detailed answer! I am very grateful to everyone who responded to my question. Your opinion is important to me! Thanks to all of you (Joo Hyung, Heesob Park and you Richard), I found the answer to my question :)

I don't know a lot about the Korean language but I do know something about the difference between Mandarin/Taiwanese and American/British English. I'm sorry to disagree with Heesob Park but I believe the difference between North Korean and South Korean languages is much closer to the difference between American English and British English. Americans and Britons can communicate with perhaps 1% difference in the spelling of some words and the definition of some words/phrases. The grammar is basically the same. Someone who speaks only Mandarin cannot understand someone who speaks Taiwanese and vice versa. Perhaps Heesob Park was thinking of the difference between Mandarin as spoken in Beijing and Mandarin as spoken in Taiwan.