2.1. Some SQL Formalities
A database engine is the part of an RDBMS that actually stores and retrieves data to and from the data files. The database engine is not very useful unless you can send SQL (Structured Query Language) commands to it and receive the results from those SQL commands (if any).
SQL (Structured Query Language)
The industry-standard database language used to query and manipulate the data, structures, and permissions in a relational database.
NOTE
"SQL" is usually pronounced "sequel," but if you refer to "S-Q-L" in a conversation with other database developers and DBAs, they will certainly know what you're talking about!
It is also important to separate the SQL commands from the command processor itself. For example, Oracle's SQL*Plus client tool (available on virtually any platform that the Oracle server itself runs on) has a number of other "builtin" commands that look like SQL commands but operate only within the SQL*Plus environment; these are called SQL*Plus commands. A SQL*Plus command may actually send many SQL commands to the Oracle server.