As far as I know it's disagreement due to slavery, but even after it racism was still a problem. What were true reasons for war and its results?
Yes, the Civil War was about slavery.
There is always more to say, of course (though, nonetheless: it was about slavery), but while you are waiting for fresh answers to your query, you might like to go into more depth with these responses from our FAQ:
The Civil War: slavery or states' rights? (Slavery.)
Why did the Civil War happen? Couldn't it have been worked out diplomatically? with /u/Georgy_K_Zhukov