Deactivate User:

• Deactivating a user in Salesforce means that user will not be deleted from the system but will no longer be able to log in to Salesforce and their records can be transferred to another user. They cannot be part of workflows or part of any automated processes.
• Deactivating a user will be available user licenses available for use in your organization.
• Deactivated users lose access to any records that were manually shared with them, or records that were shared with them as team members. Users higher in the role hierarchy relative to the deactivated users also lose access to those records. However, you can still transfer their data to other users and view their names on the Users page.

Freeze User:

• Freezing a user in Salesforce means that only stops the user from being able to login.
• In some cases, you can’t immediately deactivate a user (such as when a user is selected in a custom hierarchy field or a user that’s assigned as the sole recipient of a workflow email alert). To prevent users from logging into your organization while you perform the steps to deactivate them, you can freeze user accounts.
• Freezing user accounts doesn’t make their user licenses available for use in your organization.