A brief description of the methodology of construction, contents and usage of the Planck Early Release Compact Source Catalogue (ERCSC),
including the Early Cold Cores (ECC) and the Early Sunyaev-Zeldovich (ESZ) cluster catalogue is provided. The catalogue is based on data that
consist of mapping the entire sky once and 60% of the sky a second time by Planck, thereby comprising the first high sensitivity radio/submillimetre
observations of the entire sky. Four source detection algorithms were run as part of the ERCSC pipeline. A Monte-Carlo algorithm based on the
injection and extraction of artificial sources into the Planck maps was implemented to select reliable sources among all extracted candidates such
that the cumulative reliability of the catalogue is ≥90%. There is no requirement on completeness for the ERCSC. As a result of the Monte-Carlo
assessment of reliability of sources from the different techniques, an implementation of the PowellSnakes source extraction technique was used
at the five frequencies between 30 and 143 GHz while the SExtractor technique was used between 217 and 857GHz. The 10σ photometric flux
density limit of the catalogue at |b| > 30◦ is 0.49, 1.0, 0.67, 0.5, 0.33, 0.28, 0.25, 0.47 and 0.82 Jy at each of the nine frequencies between 30
and 857 GHz. Sources which are up to a factor of ∼2 fainter than this limit, and which are present in “clean” regions of the Galaxy where the sky
background due to emission from the interstellar medium is low, are included in the ERCSC if they meet the high reliability criterion. The Planck
ERCSC sources have known associations to stars with dust shells, stellar cores, radio galaxies, blazars, infrared luminous galaxies and Galactic
interstellar medium features. A significant fraction of unclassified sources are also present in the catalogs. In addition, two early release catalogs
that contain 915 cold molecular cloud core candidates and 189 SZ cluster candidates that have been generated using multifrequency algorithms are
presented. The entire source list, with more than 15000 unique sources, is ripe for follow-up characterisation with Herschel, ATCA, VLA, SOFIA,
ALMA and other ground-based observing facilities
