What is the difference between unique and external fields?

Hello Saloni,

External Fields:

An external ID is a custom field that has the External ID attribute, meaning that it contains unique record identifiers from a system outside of Salesforce. When you select this option, the Data Import Wizard detects existing records in Salesforce with external IDs that match those in the import file.

Unique Fields :
This is a setting for the field that will prevent you from using the same value in multiple records for the unique field. So if I create a 5 character text field and make it unique, and I create a record with the value "12345" I will not be able to create another record with that same value in the unique field. If I try to do so, I will get an error saying that the value is already in use.