Our understanding of the structure, composition and evolution of galaxies has strongly improved in the last decades, mostly due to new results based on large spectroscopic and imaging surveys. In particular, the nature of ionized gas, its ionization mechanisms, its relation with the stellar properties and chemical composition, the existence of scaling relations that describe the cycle between stars and gas, and the corresponding evolution patterns have been widely explored and described. More recently, the introduction of additional techniques, in particular Integral Field Spectroscopy, and their use in large galaxy surveys, have forced us to re-interpret most of those recent results from a spatially resolved perspective. This review is aimed to complement recent efforts to compile and summarize this change of paradigm in the interpretation of galaxy evolution. In particular we cover three particular aspects not fully covered in detail in recent reviews: (i) the spatially resolved nature of the ionization properties in galaxies and the confusion introduced by considering just integrated quantities; (ii) the nature of the global scaling relations and their relations with the spatially resolved ones; and (iii) the dependence of the radial gradients and characteristic properties of the stellar populations and ionized gas on stellar mass and galaxy morphology. To this end we replicate published results, and present novel ones, based on the largest compilation of IFS data of galaxies in the nearby universe to date.
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