SQL is the underlying language for all database programming.
PL/SQL is tightly integrated with SQL, usually for code running inside the database such as triggers,
stored procedures and functions, and PL/SQL packages.
SQL*Plus is the command-line tool for running SQL and compiling PL/SQL.
In addition to the individual books for each environment, there is an
online quick reference that combines all three environments.
PL/SQL Web Toolkit Reference