  This is a pedagogical introduction to the main concepts of the sterile
neutrino - a hypothetical particle, coined to resolve some anomalies in
neutrino data and retain consistency with observed widths of the $W$ and $Z$
bosons. We briefly review existing anomalies and the oscillation parameters
that best describe these data. We discuss in more detail how sterile neutrino
can be observed, as well as the consequences of its possible existence. In
particular, we pay attention to a possible loss of coherence in a model of
neutrino oscillations with sterile neutrinos, where this effect might be of a
major importance with respect to the 3$\nu$ model. The current status of
searches for a sterile neutrino state is also briefly reviewed.
