Grant Fritchey shares some thoughts:
The whole idea behind a database is the ability to persist the data. You want your inventory of widgets to get stored so you can look at it later. That means writing out to disks. However, what is writing to disk and where is it being written? Unlike SQL Server which has one (or more) big file for all data, PostgreSQL has a collection of a large number of files. There is a methodology and structure to these files that you need to understand in order to later understand how the data gets written to and retrieved from these files.
While we’re going to be very focused on file, page, folder, etc., throughout this article, that’s just part of the physical nature of persisting your data. What is being persisted is still the logical information you’re most interested in – rows and columns. I just wanted to emphasize the distinction between the two here.
Click through to see how PostgreSQL stores information.