  We describe and discuss the selection procedure and statistical properties of
the galaxy sample used by the Calar Alto Legacy Integral Field Area Survey
(CALIFA), a public legacy survey of 600 galaxies using integral field
spectroscopy. The CALIFA "mother sample" was selected from the Sloan Digital
Sky Survey (SDSS) DR7 photometric catalogue to include all galaxies with an
r-band isophotal major axis between 45" and 79.2" and with a redshift 0.005 < z
< 0.03. The mother sample contains 939 objects, 600 of which will be observed
in the course of the CALIFA survey. The selection of targets for observations
is based solely on visibility and thus keeps the statistical properties of the
mother sample. By comparison with a large set of SDSS galaxies, we find that
the CALIFA sample is representative of galaxies over a luminosity range of -19
> Mr > -23.1 and over a stellar mass range between 10^9.7 and 10^11.4Msun. In
particular, within these ranges, the diameter selection does not lead to any
significant bias against - or in favour of - intrinsically large or small
galaxies. Only below luminosities of Mr = -19 (or stellar masses < 10^9.7Msun)
is there a prevalence of galaxies with larger isophotal sizes, especially of
nearly edge-on late-type galaxies, but such galaxies form < 10% of the full
sample. We estimate volume-corrected distribution functions in luminosities and
sizes and show that these are statistically fully compatible with estimates
from the full SDSS when accounting for large-scale structure. We also present a
number of value-added quantities determined for the galaxies in the CALIFA
sample. We explore different ways of characterizing the environments of CALIFA
galaxies, finding that the sample covers environmental conditions from the
field to genuine clusters. We finally consider the expected incidence of active
galactic nuclei among CALIFA galaxies.
