  The Serpens cloud has received considerable attention in the last years, in
particular the small region known as the Serpens cloud core where a plethora of
star formation related phenomena are found. This review summarizes our current
observational knowledge of the cloud, with emphasis on the core. Recent results
are converging to a distance for the cloud of ~ 230 +- 20 pc, an issue which
has been controversial over the years. We present the gas and dust properties
of the cloud core and describe its structure and appearance at different
wavelengths. The core contains a dense, very young, low mass stellar cluster
with more than 300 objects in all evolutionary phases, from collapsing gaseous
condensations to pre-main sequence stars. We describe the behaviour and spatial
distribution of the different stellar populations (mm cores, Classes 0, I and
II sources). The spatial concentration and the fraction number of Class 0/Class
I/Class II sources is considerably larger in the Serpens core than in any other
low mass star formation region, e.g. Taurus, Ophiuchus or Chamaeleon, as also
stated in different works. Appropriate references for coordinates and fluxes of
all Serpens objects are given. However, we provide for the first time a unified
list of all near-IR sources which have up to now been identified as members of
the Serpens core cluster; this list includes some members identified in this
review. A cross-reference table of the near-IR objects with optical, mid-IR,
submillimeter, radio continuum and X-ray surces is also provided. A simple
analysis has allowed us to identify a sample of ~ 60 brown dwarf candidates
among the 252 near-IR objects; some of them show near-IR excesses and,
therefore, they constitute an attractive sample to study very young substellar
objects. (abridged)
