The mechanism An app is configuration, not a different product.
Pick one when you create the organisation, or install it later from Settings, Apps. It shows
what it will add before anything changes, then writes ordinary configuration rows into your org,
all of which you can edit or delete afterwards. It offers an update when a newer version adds
something you lack, and removing it hides what it added while every record stays.
Renames things to your words, on the web app
An app writes a terminology map onto the org and the web app reads it: Deals, Leads, Accounts, Contacts and Invoices take your words in the navigation, and the Leads pages carry the new word throughout. Other page headings keep the stock label, and the phone app does not read the map at all, even though you can install the app from it. It is a rename, not a fork: the record underneath is the same one.
Adds the fields your trade needs
Between three and six custom field definitions per app, on the entities where they belong. They show on every record page and form, on the web and the phone, filter in list views, and come back through the API like any other field.
Seeds tags and a product list
Four tags and six to eight products or services, priced and ready to put on an estimate. It is a starting point you can edit, not a locked catalogue.
Optionally fills it with sample data
Accounts, contacts, deals, tickets, tasks and leads, every row tagged so you can delete the lot in one action when you are done looking. Skip it and you get the configuration only.
One thing an app builds that you cannot see yet
Every app also defines a lead pipeline, a ticket pipeline and a task pipeline, with named
stages. The lead and ticket pipelines are drawn as boards on the web app and the phone. The
task pipeline is created and the API serves it, but neither client has a screen that
draws it today: the task board uses its own columns. It is listed here rather than
counted as a feature, because a stage board you cannot open is not one.