SQL is executed one statement at a time. PL/SQL is executed as a block of code.

SQL tells the database what to do (declarative), not how to do it. In contrast, PL/SQL tell the database how to do things (procedural).

SQL is used to code queries, DML and DDL statements. PL/SQL is used to code program blocks, triggers, functions, procedures and packages.

You can embed SQL in a PL/SQL program, but you cannot embed PL/SQL within a SQL statement.