There are many differences between spoken and written Korean - that is the Korean you'll actually hear Koreans using, and the Korean you might see written somewhere. They're the same language, but there are some fundamental differences that can make the two difficult. In order to master reading, writing, speaking and listening, you'll have to understand these major differences.
These differences include word order, grammar forms used and conjugations, verb endings, as well as vocabulary, phrases, and more. Did I miss anything? Let me know in the comments~