I will review recent results obtained by the PACS Evolutionary Probe (PEP) extragalactic survey. PEP covers popular multiwavelength fields from wide (COSMOS, 2 deg2) to deep, pencil-beam observations (e.g. GOODS-N/S) and lensing galaxy clusters at 70, 100 and 160 microns. Herschel observations break the confusion limit of previous infrared space telescopes and cover the whole flux range from ~1 mJy to 200 mJy, thus resolving more than 70% (55%) of the Cosmic IR background (CIB) at 160 microns (100 micron). Combining PACS-derived calorimetric star formation rates and ancillary datasets (e.g. high-resolution ACS imaging, deep Chandra X-ray maps and spectroscopy at several wavelengths), we perform a highly complete sampling of the properties of star forming galaxies: their SEDs, structure, extinction, timescale of star formation, and possible AGN components, across the so-called star-forming "main-sequence" from redshift z~2.5 to the local Universe. Among the results that I will present, our analyses show that at each mass and redshift galaxies on the main sequence have the largest size and are well approximated by exponential disks. A well defined correlation between structure and stellar populations is already in place since at least z~2.5. Objects off the main sequence, hosting merger-driven starbursts, represent only <5% of mass-selected star forming galaxies, thus playing a relatively minor role in the formation of stars in galaxies.