What is a database link? What is the difference between a public and a private database link? What is a fixed user database link? | Oracle 10g

 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.