App Catalog in SharePoint 2013

Nothing but a special site collection which holds all apps. (custom or third-party apps)
Users can install apps from app catalog to their sites.
Each web application can have only one app catalog site collection / one app catalog per SharePoint Online tenant
There can be multiple app catalogs in a farm.

App Catalog Creation:
1. CA --> Apps --> app management --> manage app catalog
2. Select web application from the drop down and click "Create a new App Catalog site"
3. Provide the details and click on.

App Catalog Creation For Office 365:
Sign in to Office 365 --> SharePoint admin center --> Apps --> App Catalog --> Create a new app catalog site


Below are the libraries which are created by default inside app catalog site collection

1. Apps for SharePoint
• This is the library where we upload our app so that it will be available across the web application.
2. Apps for Office
3. App Requests

Adding an app from app catalog to our site collection:

Site owner access should be there to add an app.
Navigate to Site collection --> Site contents --> add an app
Now you can select your app which you uploaded in your app catalog --> click --> Trust It

Read What is App In Sharepoint