A war fought in the United States, 1861-65, over issues such as state vs. national sovereignty and slavery vs. abolitionism. The two sides consisted of the United States of America (known more commonly as the Union and consisting of fifteen northern states, plus California) and the newly formed Confederate States of America, consisting of eleven southern states who stood for state's rights and slavery. The Union eventually triumphed in 1865 after a long and costly struggle on both sides, and the seceding states were eventually readmitted to the Union. The war has been called the first official modern war, as both sides incorporated telegraph communication, the use of railroads as transportation, the introduction of new military technology such as rifled muskets and ironclad warships and the doctrine of "total war," or war against civilians as well as military personnel. Finally, the war produced such iconic figures in American history as Abraham Lincoln, Robert E. Lee, Ulysses S. Grant (who later became President), Thomas "Stonewall" Jackson, Joshua L. Chamberlain and countless others.
As a side note, many today commonly believe the war was fought over slavery alone. While slavery was eventually used by the Union to justify the war, the issue of state's rights vs. national sovereignty first caused the outbreak of war. There are perhaps thousands of books written on the American Civil War, its cause and its aftermath, so I suggest you read those and get a better understanding of the subject that from what I can tell you.
The American Civil War has been called the noblest and least avoidable conflict up until that point in human history.