Neutrinos (meaning: "Small neutral ones") are elementary particles that often travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect.
Neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter Ν (nu).
Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms.
There are three types, or "flavors", of neutrinos:
muon neutrinos and
each type also has an antimatter partner, called an antineutrino.
Electron neutrinos or antineutrinos are generated whenever neutrons change into protons or vice versa, the two forms of beta decay.
Interactions involving neutrinos are generally mediated by the weak force.
Most neutrinos passing through the Earth emanate from the Sun, and more than 50 trillion solar electron neutrinos pass through the human body every second. 
Photo:"Neutrinos in the Sun"NASA Astronomy Photo of the Day, June 5, 1998. Credit: R. Svoboda and K. Gordan (LSU).
Photo-Explanation: Neutrinos are fundamental pieces of matter, along with things like electrons and quarks. But neutrinos are hard to detect. Readily produced in nuclear reactions and particle collisions, they can easily pass completely through planet Earth without once interacting with any other particle. Constructed in an unused mine in Japan, an ambitious large-scale experiment designed to detect and study neutrinos is known as Super-Kamiokande or "Super-K". Only(!) 500 days worth of data was needed to produce this "neutrino image" of the Sun, using Super-K to detect the neutrinos from nuclear fusion in the solar interior. In the image, brighter colors represent a larger flux of neutrinos.
Neutrino-News: In a tantalizing recent announcement, an international collaboration of Super-K researchers has now presented evidence that the ghostly neutrinos undergo quantum mechanical oscillations, changing their particle identities and quantum properties over time. Theorists have considered neutrinos to be massless particles but these oscillations would imply that they have a very small (but nonzero) mass. Astrophysicists are taking note because even a small mass for ubiquitous, nearly undetectable neutrinos would make them accountable for a substantial fraction of the total mass of our Universe, influencing and perhaps determining its ultimate fate!
A measurable mass for neutrinos would also make them candidates for the mysterious dark matter known to affect the motions of stars and galaxies, while proof of neutrino oscillations would be a step toward resolving the decades old Solar Neutrino Problem. Even skeptical scientists will be waiting impatiently to see if these results are independently confirmed.
(Source:NASA APOD 6/5/98.)