The riots were in response to laws which had recently been enacted by the US Congress to draft men to fight the Civil War. The law contained a provision allowing those who could pay a $300 fee to have some one else go in his place. The working class resented this clause which meant that wealthier men could avoid service.
This combined with tensions between freed blacks and recent immigrants created a tinderbox that erupted when the second lot of draftee numbers were drawn on July 13. The First lot had been peacefully drawn two days earlier on Saturday, July 11.
The riots continued until Thursday, July 17 when police, and various regiments of the New York State militia were able to quell the riots.
It is estimated that between 2,000 and 8000 people were injured, with those at the higher end of the spectrum estimiating 2,000 killed. Somewhere between $1-5 million was done in property damage ($15-75 million in 2011 dollars).