Of the various kinds of theme files, you'll likely interact with Templates the most. They contain the markup for all of the standard pages associated with a theme, like a category page, a product page, or a customer account page.
Most templates are organized by resource, which is why you see file names like "brands/show.liquid" or "products/index.liquid".
Here's a list all the templates in a theme:
This may seem a bit unusual, but these files are named based on the conventional names for various HTTP actions. In general, index corresponds to a list of things and show corresponds to an individual thing. Edit pages are where your customers can make changes to some resource. Currently, the only thing a customer can edit is their own account or another resource (e.g. addresses) related to their account.
There are several specialized template files that do not follow the structure described above.
The "index.liquid" file is the homepage of your theme. Unlike some other template files, the index file is essentially a blank slate without much structure to it.
The "cart.liquid" file lists the items currently in your customer's cart.
The "404.liquid" file is the page that appears when a user tries to navigate to a URL that does not exist.