We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, comparing UDGs in the field and in galaxy groups. We find that while field dwarfs puff up to UDGs by multiple episodes of supernova feedback (\citeauthor{dicintio17}), as satellites, they become quiescent and diffuse by ram-pressure stripping and tidal puffing up. The field and group UDGs share surprisingly similar properties, independent of distance from the host-halo center. Their dark-matter halo mass is in a characteristic narrow mass range $\log (M_{\rm vir}/\Msun) = 10.5 \pm 0.6$, with a typical spin parameter, and a flat dark-matter core that dominates within the stellar half-mass radius. Their stellar components tend to have a prolate shape and do not show strong rotation although the surface density profiles have a S\'ersic indices $\sim 1$ indicative of disky morphology. Ram-pressure removes the gas from the group UDGs when they are at pericenter, quenching star formation in them and making them redder. This generates a color/star-formation-rate gradient with distance from the host-halo center, as observed in clusters. We find that $\sim20$ per cent of the field UDGs survive as satellite UDGs after falling into a massive halo. In addition, less diffuse field galaxies on highly eccentric orbits can become UDGs near pericenter due to tidal puffing up, which contribute an almost equal amount to the group-UDG population. We interpret our findings using simple toy models. We show that ram-pressure is more important than tides for stripping gas, and that the tidal energy deposited to the bound part of a satellite over a full orbit can indeed be significant (up to $\sim50$ percent of the binding energy of the satellite), although for tidal heating to influence the stellar component, the orbit needs to be eccentric, with $j/j_{\rm c}(E)\la0.4$.