South Korean vs. North Korean?

They're just different dialects, but they are really quite different.

Native Korean speakers can understand it without a lot of trouble, but it's very hard for me because they have lots of words that are very different from the Korean spoken in the South.

The Southern dialects of Korean tend to have a lot of loan words deriving from English, Japanese and Chinese.

In the Northern dialects the vast majority of the loanwords come from either Chinese or Russian, since English is the language of the 'imperialists' the North Koreans got rid of all the English loanwords and made their own new ones ages ago.

Even though I'm not a Korean native speaker, I can identify the Northern dialects immediately.

they're just different dialects. They can understand each other just fine (think British English vs American English or something of that sort).