These systems show a rich variety of physical phenomena as
quantum phase transitions, spin liquid behavior or exotic type of
magnetic ordering. By using different analytical and numerical
approaches as exact diagonalization, the 'decoupled-cell' quantum
Monte-Carlo method or variational wave function calculations the
ground-state, the low energy spectra, spin-spin correlations and
thermodynamic properties as entropy or specific heat has been analysed.
Special interest has been devoted to the question which kind of
magnetic order is present in the ground-state of low-dimensional spin
systems. It has been shown that strong quantum fluctuations in
connection with frustration, dimensionality, coordination number and
other factors influence the magnetic ordering in quantum spin systems in
different ways.
