I walked the blocks from E 87th - E 96th (about 9 miles), finishing off the Upper East Side ("UES"). There's nothing significant about E 96th St. that makes it a boundary, but everything north of there is generally considered East Harlem.
There are two main sub-neighborhoods in the upper blocks of the UES; Carnegie Hill (E 86th St - E 96th St. east of 3rd Ave.) and Yorkville (E 79th St - E 96th St. west of 3rd Ave.)
Carnegie Hill is named for the Andrew Carnegie Mansion on E 91st St. and 5th Avenue, which today houses (or will house, looked like a lot of construction going on) the Smithsonian Museum of Design. The 5th Ave. side of the neighborhood is lined with many other beautiful mansions, but you probably wouldn't recognize the names if you're not a big early-20th century New York finance history buff.
The Yorkville neighborhood was mostly farm land until the New York and Harlem railroad was built in the 1830s and a small town grew up around where the 86th St. Station is today. It was a largely German/central European community in the late 1800's and early -mid 1900's. I don't really have a third sentence to finish this paragraph.