A database link allows you to make a connection with a remote database, Oracle or not, and query tables from it, even incorporating those accesses with joins to local tables.
A private database link only works for, and is accessible to the user/schema that owns it. A global one can be accessed by any user in the database.
A fixed user link specifies that you will connect to the remote db as one and only one user that is defined in the link. Alternatively, a current user database link will connect as the current user you are logged in as.
As you prepare for your DBA Interview, or prepare to give one, we hope these questions provide some new ideas and directions for your study. Keep in mind that there are a lot of directions an interview can go. As a DBA emphasize what you know, even if it is not the direct answer to the question, and as an interviewee, allow the interview to go in creative directions. In the end, what is important is potential or aptitude, not specific memorized answers. So listen for problem solving ability, and thinking outside the box, and you will surely find or be the candidate for the job.