PL/SQL is an extension of SQL that allows procedural logic to be added to SQL statements and queries. Key features of PL/SQL include blocks, exceptions, cursors, procedures, functions, packages, and triggers. A PL/SQL block contains a declare section to define variables, an executable section to contain program logic, and an optional exception section. Exceptions can be predefined, non-predefined, or user-defined. The document provides examples of handling different types of exceptions within PL/SQL blocks to avoid abnormal program termination.