Shinagawa is the station between Tamachi and Oosaki.  It is mainly a business district, so you will find many office buildings and quite a few hotels as well.  Shinagawa is one of the larger train stations in the sense that it is a transfer point for many different train lines rather than a major destination in and of itself, and the station itself is quite large.  Besides the typical JR lines, Shinagawa also has Keikyu lines and the Shinkansen (Bullet Train).

Trivia:  There is a Shinagawa ward, but Shinagawa station is actually in Minato ward, not Shinagawa ward (Note that most people mean the station when they say something like "Meet me at Shinagawa").  Also, the distance between Shinagawa and Tamachi is the longest in the Yamanote line.

Shops and such:  The second floor inside the concourse has a number of shops, including a book store, a stationary shop, a doughnut shop, and a few restaurants.  Outside the gates there is a Starbucks' coffee shop with a unique layout and view.  If you go up the escalator, near the Starbucks', you will find a large supermarket, and a few restaurants and bars.

There are two major exits from Shinagawa JR; Konan, and Takanawa.  The Konan exit leads to the majority of the office buildings.  That area also has a Pachinko (gambling) place, and a few bars and such.  The Takanawa exit leads to where most of the hotels are, as well as "Wing Takanawa", a small shopping mall.  The most famous of the hotels is the Shinagawa Prince hotel, which boasts a movie theater, a bar with an excellent view, and more inside.  If you walk more than a few blocks away from either exit, you will see your surroundings quickly deteriorate into areas with nothing much to see.