What are the system fields in Salesforce ?

 System fields are Read-only fields that provide information about a record from the system, like when the record was created or when it was last changed. 

For example, Created Date, Last Modified by (User Name), Last Modified Date etc.

There are some fields which are called audit fields as well. For example, Last Modified by (User Name), Last Modified Date etc. The reason is that these fields can be used to find details like when and by whom a record has been updated.