You can use apps to enhance your Shopify admin or add new features. There are two different types of apps that you can add to your Shopify store: public and custom.
There are two types of public apps, listed and unlisted. Both listed and unlisted apps must pass Shopify's app review process. And both can be installed from the Shopify App Store.
If an app is unlisted then it doesn't appear in Shopify App Store search results or categories. Instead, you can install it from the app's App Store listing or directly from the developer's website.
Custom apps can be used to add features to your Shopify admin or to extend your online store to other platforms using custom storefronts. Custom apps are built exclusively for your Shopify store and aren't listed on the Shopify App Store.
An external app developer can make a custom app for you without needing API access to your store and access to your Shopify admin. You can install a custom app by clicking an install link provided to you by the app's developer.
If you're working with a developer that you trust, or if you're making your own custom app, then you can also create and install a custom app in the Shopify admin.
Draft apps are used by Shopify Partners during development, and can only be installed on development stores for testing purposes. Installing a draft app on a development store disables store transfer permanently.
Draft apps can only be installed on your store after Shopify review, or once they're converted to a custom app.