A neutrino is a light electrically-neutral elementary particle, specifically a fermion, i.e., has half-integer spin, in this way like electrons, quarks and nucleons.
Neutrinos are very common (estimated they average 340 per cubic centimeter throughout the universe, for a total of 1.2×1089) but do not interact very much with photons or known matter, thus are virtually invisible and pass through solids like the Earth.
Three types of neutrinos are known (aka neutrino flavors, neutrino families, neutrino species). It is also known that they come in three masses, but you cannot determine a specific neutrino's type and its mass as well (an uncertainty imposed by its quantum mechanics in a similar manner to the Heisenberg uncertainty principle). Some constraints are known regarding the ratios of the three masses, and it has been established that each mass is very likely to be less than 0.12 eV/c2. Cosmology-related astronomical observations have as a goal, to determine neutrinos' mass given the resulting effect on the universe.
Speculation exists that there might be a fourth species, currently termed a sterile neutrino, sterile in that it does not react in the manner currently used to investigate neutrinos. Also, that there might be more than one, e.g., a set of three more, all sterile. Some theories suggest it (or one of the three) could have sufficient mass to constitute dark matter, i.e., a WIMP.
Neutrinos below a certain mass are "light" (light neutrinos). Given the upper limit established on the mass of observed neutrinos, they are all in this category. If there are more massive neutrinos such as the sterile neutrino, these would be considered heavy neutrinos. Cosmological observation is considered to have eliminated a middle ground, i.e., there is a gap between the masses of known neutrinos and the possible heavy neutrinos.
Some particle interactions (e.g., created in particle accelerators) and some observed cosmological data are dependent upon the count of light neutrino species. The results of their measurement and analysis produce a likely number of neutrino species, termed the effective number of neutrino species (Neff). For example, a particular experiment and analysis might produce the result 2.9. Along with error bars, this might suggest 3 as the number.