The Settings area configures how the site and store behave. It's organized in two places:

Site settings

Per-site options edited from Settings → Sites (one entry per site in multisite setups), grouped in tabs:

Global settings pages

Page Purpose
Email Mail transport (SMTP)
Notifications Event → provider delivery matrix
Order statuses / payment / shipping / subscription statuses Status vocabularies
Post types / product types Content modeling
Tax classes, tax rates, tax types, taxonomies Commerce configuration
Subscription plans Recurring billing schedules
Discounts Automatic cart discounts
Checkout / Shipping Store flow configuration
Length classes / weight classes Measurement units

Changes under Site apply to that site only; global settings pages affect the whole installation.