Neutrinos are the most abundant elementary particles in the Universe. There are several hundred neutrinos in each cubic centimetre of the Cosmos, moving with speeds close to the speed of light. And although they are everywhere around us, we don't notice them since they are the most elusive, tiniest and lightest elementary particles that we know of. Until just a few years ago people thought that neutrinos were massless, but with the discovery of neutrino oscillations (change from one neutrino type to another (and back), over long distances) it was confirmed that neutrinos do indeed have very small, but non-zero mass. The phenomenon of 'oscillations', not seen in any other elementary particle, leads to several important conclusions: mass is a property that can be attributed to all matter, neutrinos add missing mass to the dark matter (at least part of it), neutrinos may have played a role in catalysing the primordial density fluctuations which eventually grew into galaxies and they can help us understand why elementary particles have the masses that they do. We will undertake a 50 year long journey through neutrino physics, from the discovery of neutrinos to the discovery of neutrino oscillations, which can only happen if they have mass. Finally, the KamLAND experiment will be described in detail, as an example, of a terrestrial long baseline experiment designed to study neutrino oscillations and their properties.