config/
Contains... configuration of course!. You will configure routes, services and packages.
src/
All your PHP code lives here.
templates/
All your Twig templates live here.
bin/
The famous bin/console file lives here (and other, less important executable files).
var/
This is where automatically-created files are stored, like cache files (var/cache/) and logs (var/log/).
vendor/
Third-party (i.e. "vendor") libraries live here! These are downloaded via the Composer package manager.
public/
This is the document root for your project: you put any publicly accessible files here.