Johnny Booth was a popular actor, especially in the South (although his brother, Edwin, was one of the most famous American actors of the day). A former soldier and passionate supporter of the Confederate cause, Wilkes devised a ludicrous plan to kidnap Abraham Lincoln and exchange his freedom for that of thousands of Confederate prisoners of war. That plan failed, so he turned to assassination. On April 14, 1865, he yelled "SIC SEMPER TYRANIS" (the state motto of Virginia, Latin for Thus, Always Tyrants) and fatally shot Lincoln in Ford's Theatre during a performance of Our American Cousin. He escaped by jumping off of the balcony, braking his leg in the process, and and fled with his confederates to Virginia. Three days later, he was found dead (was killed or committed suicide) in a tobacco barn. Allegedly, he had letter to newspapers explaining his motives when he died. If it existed, the letter was never published.
For more detailed information on John Wilkes Booth, see the Insider's Guide to the Civil War