  The Palomar Transient Factory (PTF) Orion project is an experiment within the
broader PTF survey, a systematic automated exploration of the sky for optical
transients. Taking advantage of the wide field of view available using the PTF
camera at the Palomar 48" telescope, 40 nights were dedicated in December
2009-January 2010 to perform continuous high-cadence differential photometry on
a single field containing the young (7-10Myr) 25 Ori association. The primary
motivation for the project is to search for planets around young stars in this
region. The unique data set also provides for much ancillary science. In this
first paper we describe the survey and data reduction pipeline, and present
initial results from an inspection of the most clearly varying stars relating
to two of the ancillary science objectives: detection of eclipsing binaries and
young stellar objects. We find 82 new eclipsing binary systems, 9 of which we
are candidate 25 Ori- or Orion OB1a-association members. Of these, 2 are
potential young W UMa type systems. We report on the possible low-mass (M-dwarf
primary) eclipsing systems in the sample, which include 6 of the candidate
young systems. 45 of the binary systems are close (mainly contact) systems; one
shows an orbital period among the shortest known for W UMa binaries, at
0.2156509 \pm 0.0000071d, with flat-bottomed primary eclipses, and a derived
distance consistent with membership in the general Orion association. One of
the candidate young systems presents an unusual light curve, perhaps
representing a semi-detached binary system with an inflated low-mass primary or
a star with a warped disk, and may represent an additional young Orion member.
Finally, we identify 14 probable new classical T-Tauri stars in our data, along
with one previously known (CVSO 35) and one previously reported as a candidate
weak-line T-Tauri star (SDSS J052700.12+010136.8).
