kintone provides the following three features to integrate data in multiple apps: Lookup, App Action, and Related Records.
The three features differ in their benefits. Choosing which one to use depending on your use cases, you can use kintone even more efficiently.

Lookup is a feature that allows you to retrieve data from a different app.
Lookup enables you to search and copy necessary data in other apps when you enter data in an app's record. This feature reduces the effort required for data entry and prevents typing errors.
For example, you can add a Lookup field to an Activity History app to reference a Customer Management app. The Lookup field can copy customer information from the Customer Management app to the Activity History app. The copied information is displayed with a link to the data source.
For details on lookups, refer to Lookup Settings.

To copy data between apps, you can also use App Action, which is described below.

App Action is useful when you want to copy existing data to a specified app.
For example, you can add an app action to a Customer Management app to copy the customer information to a Lead Management app or an Activity History app.
App Action differs from Lookup in that an action is added to a data source app, from which data is copied. When you add an app action to the Customer Management app, you can easily copy customer information from the Customer Management app to the Activity History app by clicking the app action button.
For details on App Actions, refer to Overview of App Actions.

Target apps of app actions can have Lookup fields to combine both features.

Related Records is a feature that lists records that are related to the current record. Related Records differ from Lookup and App Action in that a Related Records field just displays data but does not copy anything.
For example, this feature enables a Customer Management app to display a list of leads or sales activities that are related to a specific customer.
You can also implement a reverse reference by combining Lookup and Related Records.
For details, see the Listing Related Records.