Shinagawa Station is the first major station south of Tokyo Station and is a major interchange for trains operated by JR East, JR Central and Keikyo. The Tokaido Shinkansen and other trains to the Miura Peninsula, Izu Peninsula and the Tokai region of Honshu pass through here. It is known as the southern gateway to Tokyo. Despite its name, it is not located in Shinagawa Ward but just to the north in Minato Ward.
Today, Shinagawa is a central train hub for Tokyo, possibly second only to Tokyo Station itself in the number of trains you can catch here. It is also a center of "European" style businesses and tourism, so you will see more English than Japanese on signs, and more Italian and American restaurants than Japanese ones.
(reference: Wikipedia, Wikitravel)