Computational science uses computation in order to advance knowledge in fields ranging from physics to medicine to the social sciences and humanities. Computational science is not computer science or mathematics but it draws on the tools both of these disciplines provide in order to solve problems in other fields.
In physics, computation has been referred to a third branch of physics, complimenting both experiment and theory. Chemists use computational science to understanding chemical bonding on the atomic level as well as to work out the structure of viruses such as HIV.
Computational science plays a major role in modern industry. Modern airliners would not be possible with out detailed simulations of wing designs and the stresses on the airframe. Pharmaceutical companies use computational science to help speed up the discovery of new drugs. Tire manufacturers use computational science to design tires and to model how tires wear.
In medicine computational science is used to help decipher the human genome. In the humanities computational science is used for linguistic studies and for projects such as reassembling the Thera frescoes where computer algorithms are being developed to help match fragments of the frescoes (see http://gfx.cs.princeton.edu/proj/thera/ for details).