Hi Rajesh Swarnkar,
Thank you for posting your query on Microsoft Q&A!
We have a doc here on Device identities with links to more in depth information for both - https://learn.microsoft.com/en-us/entra/identity/devices/overview
Generally registered devices would be users personal devices, mobile phones or laptops etc.. they log into the device with their personal credentials.
An Entra ID joined device is connected to your organization, and users can log into the devices with their work account.
Do let me know if you have any further queries and I would be happy to help.
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