We can differentiate these on basis of following key points:
SQL stands for Structured Query Language whereas PL/SQL stands for Programmable Structured Query Language.
In SQL, we can execute single command or query at a time. The query can be and DDL (Data Definition Language) or DML (Data manipulation language). DDL is used to add, remove or modify tables(db objects). DML is used to Insert, update or delete data from tables.
On the other hand, PL/SQL is bloc of commands or code which helps us to accomplish multiple task at a time. It can consist of several SQL queries, loops, functions. So many tasks are performed at a time here.
I found useful resource at- Difference Between sql and plsql: Insight with Example Query