The mechanism of the generation of the neutrino mass term at present is unknown. We also do not know if neutrinos with definite masses are Dirac or Majorana particles. However, as we will see later, it was established by all existing experimental data that neutrino mixing takes place. The unitary mixing matrix U is the object of central interest of theory and experiment. Here we will consider the general properties of the matrix U in the Dirac and Majorana cases.