Themes
Themes are what give the Shopify front end its look and feel. To preview a theme go to Online store from the admin panel, select Actions > Preview on the theme you want to see.
All themes come with settings that are particular to that theme. Updating a setting in one theme will not affect the same settings in another. These settings could be colors, fonts, images, or text etc. To edit a theme's settings, select Customze for that theme.
On the customize screen, you will see a panel on the left with the theme settings. There are two areas here, Sections and General Settings
Theme sections are settings for a particular page in shopify, at first you will be presented with the sections for the home page. Sections can be re-arranged by dragging them in order. Selecting a section will show the settings for that section
Sections can have blocks that are like sections for sections. These can also be re-arranged by dragging
Some of the other page types also have sections (depending on the theme). The only way to see these sections is by navigating to that page type in customize mode and checking the side panel
General settings are usually site wide settings. These include things like colours, typography, and checkout settings etc.
Most themes come with language files. These define terms used throughout the theme and have the same fields for different languages. From the theme page select Actions > Edit languages