NASA's Discovery Program is a set of low-cost, focused scientific space missions that are designed to explore the Solar System, generally, but not solely, centered on planetary bodies. However, for example, other Discovery Program missions include “Genesis,” a mission to collect charged particles from the solar wind, or “Stardust,” a mission that collected dust particles from a comet.
The Discovery Program was implemented in 1992 to fulfill the goal of developing faster and better and cheaper unmanned missions to explore the Solar System. As the program’s website says: “NASA's Discovery Program gives scientists the opportunity to dig deep into their imaginations and find innovative ways to unlock the mysteries of the solar system. . . For the first time, scientists and engineers were called on to assemble teams and design exciting, focused planetary science investigations . . .” http://discovery.nasa.gov/program.cfml
The driving paradigm of the Discovery Program is to ask for plans for a mission from non-NASA sources. Then NASA puts together a team made up of representatives from academia, NASA, other federal agencies and private industry, all led by the principal investigator. The principal investigator initiates the mission by setting the scientific objectives and developing the instrument package. The various team members then bring their expertise to see the mission successfully accomplished on time and on budget.
The Discovery Program’s most famous missions have to be the Mars Pathfinder mission which explored the surface of Mars in the late nineties and the ongoing Kepler space telescope designed to find extra-solar planets. Another ongoing mission is Mercury Surface, Space Environment, Geochemistry and Ranging, also called “MESSENGER,” a robotic spacecraft currently orbiting Mercury and exploring that planet. Also ongoing is the “DAWN” mission, a probe that is designed to explore the dwarf planets Vesta and Ceres.
One standalone mission is currently in development: the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, called "InSight." This mission is designed to study the interior structure and composition of the planet Mars to advance the understanding of how terrestrial planets are formed. Other missions that were under consideration but not accepted were the Comet Hopper, or "C-Hopper" which would have landed on a comet many times to study the changes of a comet as it orbited the Sun; and the Titan Mare Explorer, which would have landed a probe on the seas of Titan, a moon of Saturn.
Also in development is “Strofio,” a mass spectrometer which will be aboard the European Space Agency’s Mercury Planetary Orbiter. “Strofio” is designed to study the atmosphere of Mercury.