A neutrino from the sun hits an electron in the water, transferring a lot of energy to the electron. The high energy electron moving through the water creates visible light. The light is detected by photodetectors that line the water tank.
Result: Neutrino reactions detected, but not as many as expected based on the theoretical calculations.
The Sudbury Neutrino Observatory in Canada is similar. It can detect neutrinos in more ways because it is filled with D2O instead of H2O, where D denotes deuterium (a proton + a neutron).
SNO has found that the total number of neutrinos coming from the sun is greater than the number of electron neutrinos. Thus some of the electron neutrinos must have changed into muon or tau neutrinos. (Probably into muon neutrinos.)
Updated 22 Octobber 2007
Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA