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The PAU (Physics of the Accelerating Universe) project goal is the study of dark energy with a new photometric technique aiming at obtaining photo-z resolution for Luminous Red Galaxies (LRGs) roughly one order of magnitude better than current photometric surveys. To accomplish this, a new large field of view camera (PAUCam) has been built and commissioned at the William Herschel Telescope (WHT). With the current WHT corrector, the camera covers ~1 degree diameter Field of View (FoV). The focal plane consists of 18 2kx4k Hamamatsu fully depleted CCDs, with high quantum efficiency up to 1 μm. To maximize the detector coverage within the FoV, filters are placed in front of the CCD's inside the camera cryostat (made of carbon fiber material) using a challenging movable tray system. The camera uses a set of 40 narrow band filters ranging from ~4400 to ~8600 angstroms complemented with six standard broad-band filters, ugrizY. Here, we describe the camera and its first commissioning results. The PAU project aims to cover roughly 100 square degrees and to obtain accurate photometric redshifts for galaxies down to iAB ~ 22:5 detecting also galaxies down to iAB ~ 24 with less precision in redshift. With this data set we will obtain competitive constraints in cosmological parameters using both weak lensing and galaxy clustering as main observational probes.