Azure Active Directory (AD) Join and Azure AD registration are two different methods to connect a device to Azure AD. Here's a comparison of the two:

Azure AD Join:

Azure AD Join allows you to join a Windows device to Azure AD, meaning that users can sign in to the device using their Azure AD credentials.

The device becomes a trusted device in Azure AD, which enables users to access company resources such as email, OneDrive, and other cloud services.

The device must be running Windows 10 and the user must have admin rights on the device to perform the Azure AD Join.

Azure AD Join also allows you to use features like device-based conditional access, which allows you to set policies that require a device to meet certain security requirements before granting access to company resources.

Azure AD Registration:

Azure AD Registration allows you to register a device with Azure AD, meaning that the device is known to Azure AD but the user is not required to sign in to the device using their Azure AD credentials.

The device is not a trusted device in Azure AD and users cannot access company resources using the device.

Azure AD Registration can be performed on any device running Windows 8.1 or later, or Mac OS X Mavericks 10.9 or later.

Azure AD Registration is useful for scenarios where you want to manage the device with Intune, but the device is not used to access company resources.

In summary, Azure AD Join is a more secure and feature-rich option that allows users to access company resources, while Azure AD Registration is a lighter-weight option that enables device management but does not provide access to company resources.