  The low-energy structure and the thermodynamic properties of ferrimagnetic
Heisenberg chains of alternating spins $S$ and $s$ are investigated by the use
of numerical tools as well as the spin-wave theory. The elementary excitations
are calculated through an efficient quantum Monte Carlo technique featuring
imaginary-time correlation functions and are characterized in terms of
interacting spin waves. The thermal behavior is analyzed with particular
emphasis on its ferromagnetic and antiferromagnetic dual aspect. The extensive
numerical and analytic calculations lead to the classification of the
one-dimensional ferrimagnetic behavior according to the constituent spins: the
ferromagnetic ($S>2s$), antiferromagnetic ($S<2s$), and balanced ($S=2s$)
ferrimagnetism.
