SQL is a language: Structured Query Language. It has been 'invented' by IBM.
SQL*Plus is a command line tool from Oracle Corp. to use the SQL language for a given database.
SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and reporting tool. Its a command line tool that allows user to type SQL commands to be executed directly against an Oracle database. SQL is a language used to query the relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set options, Edit SQL commands and PL/SQL.