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I've recently stumbled upon this great site after having floated around the internet for the last few weeks trying my hand at creating vector maps for my garmin, so I apologize if this is an ignorant question:

I am in the dark about postGIS and postgreSQL. As far as I know they are utilities to convert data to specific filing structures in order to provide for more efficient GIS applications. Looking over the garmin maps tutorial, the programs are used at some parts but not others. My question is basically WHAT are postGIS and postgreSQL (i.e., how can it change a given data set), and more importantly WHY are they so advantageous to use?

Thank you!