Installation art is otherwise known as land art and is three-dimensional pieces that can range from pieces filling a small room to an entire warehouse. It is usually created for a specific space and most of the time incorporates materials from the sight. The installations can be inside or outside and be of just about anything. I find myself having difficultly describing this type of art but it basically is filling up a large space with items that evoke emotions, associations, and thought. An example of installation art is Damien Hirst's Pharmacy which is basically a room sized pharmacy with four bottles of food coloring and water. The bottles represent the four elements earth, air, fire, and water. You can assume this for yourself because they are green, yellow, red, and blue. There is also an insect-o-cutor for insects that possibly symbolizes a God without mercy. Although the shelves are stocked with packages of medicine there is no real medicine actually involved in the piece.
Another installation artist is Rachel Whiteread who is a British artist known for her sculptures. She is probably best known for her work Ghost where she made a cast out of the negative space of a victorian home. Also her work Embankment which was displayed at the Tate Modern was around 14,000 translucent white boxes in a large room which is made to feel almost like a trip to Arctic because that was her inspiration.