Trocadéro can be understood to be either the plaza that stands across the river from the Eiffel Tower, or as the overall hill that includes more than just the plaza - two theaters, a restaurant, and several museums, including, making your way down the hill, the musée de la marine, musée de l'homme, the "cinéaqua" aquarium, the city's modern art museum. Across the way is the palais Galliera, ie the fashion museum.
Note that the Galliera and the Musée de l'homme are closed for renovation, till 2011 and 2012 respectively.