The battle began when the former American frigate USS Merrimack, converted into a large and somewhat unwieldy ironclad warship called CSS Virginia by the Confederate States Navy steamed into Hampton Roads, Virginia on the morning of March 8, 1862, and set to work attempting to break the Federal blockade. Accompanied by three other rebel warships, Virginia sank USS Cumberland and forced USS Congress aground, where she later exploded when fires caused by the rebel ironclad caused her magazine to explode. While attempting to finish off the warship USS Minnesota, the ironclad found itself seriously damaged by Federal shore battery fire and retreated into the safety of Rebel controlled waters for the night.
The next morning, after undergoing repairs, Virginia returned to finish her kill of Minnesota but found her way blocked by the newly arrived USS Monitor, which the commander of the rebel ship later described as "little more than a cheesebox on a raft". After fighting for close to nine hours, both ships withdrew from the engagement, claming victory. Recent evidence has suggested an irony that the officers at the time might have found humorous; had Virginia been carrying Monitor's armament, and vice versa, the two might have done serious damage to each other.
SOURCES: The Dictionary of American Fighting Ships; I. Hogg, "Weapons of the Civil War".