Oracle allows you to query separate databases using Database Links.
You can even setup a Database Link in an Oracle database with a non Oracle database such as SQL Server or Sybase.
Creating these types of Database Links can take some time, and sometimes you just want to reference a small bit of data on another Database and use it in your query.
SQL Developer allows you define connections to multiple databases including non Oracle databases. You can query each connection separately but until now you couldn't easily query different data sources (connections) at the same time in the one statement.