What was the Lusitania and why was this event extremely significant during World war 1?

RMS Lusitania was a British ocean going passenger liner famous for her luxurious accommodation, and speed capabilities. As well as carrying people she was used to transport goods across the Atlantic (between Britain and America).
On the 1st May 1915 she left port in New York on her way to Liverpool England - she was carrying 1,959 passengers that included 159 US citizens.
On May 7th 1915 due to thick fog Captain William Thomas Turner slowed the ship down and steered a steady course. The Lusitania was hit by a torpedo 14miles of the coast of Ireland and sank with the loss of 1,198 people, including 128 Americans.
This great loss of civilian life shocked the world and caused heightened tensions between America and Germany. This swayed public opinion towards the US joining the war.