  We review the properties of low mass dense molecular cloud cores, including
starless, prestellar, and Class 0 protostellar cores, as derived from
observations. In particular we discuss them in the context of the current
debate surrounding the formation and evolution of cores. There exist several
families of model scenarios to explain this evolution (with many variations of
each) that can be thought of as a continuum of models lying between two extreme
paradigms for the star and core formation process. At one extreme there is the
dynamic, turbulent picture, while at the other extreme there is a slow,
quasi-static vision of core evolution. In the latter view the magnetic field
plays a dominant role, and it may also play some role in the former picture.
Polarization and Zeeman measurements indicate that some, if not all, cores
contain a significant magnetic field. Wide-field surveys constrain the
timescales of the core formation and evolution processes, as well as the
statistical distribution of core masses. The former indicates that prestellar
cores typically live for 2--5 free-fall times, while the latter seems to
determine the stellar initial mass function. In addition, multiple surveys
allow one to compare core properties in different regions. From this it appears
that aspects of different models may be relevant to different star-forming
regions, depending on the environment. Prestellar cores in cluster-forming
regions are smaller in radius and have higher column densities, by up to an
order of magnitude, than isolated prestellar cores. This is probably due to the
fact that in cluster-forming regions the prestellar cores are formed by
fragmentation of larger, more turbulent cluster-forming cores, which in turn
form as a result of strong external compression.
