PostgreSQL allows users to add new programming languages to be available for writing functions and procedures. These are called procedural languages (PL). In the case of a function or trigger procedure written in a procedural language, the database server has no built-in knowledge about how to interpret the function's source text. Instead, the task is passed to a special handler that knows the details of the language. The handler could either do all the work of parsing, syntax analysis, execution, etc. itself, or it could serve as "glue" between PostgreSQL and an existing implementation of a programming language. The handler itself is a special programming language function compiled into a shared object and loaded on demand.
Writing a handler for a new procedural language is described in Section 12.7. Several procedural languages are available in the standard PostgreSQL distribution, which can serve as examples.
If you see anything in the documentation that is not correct, does not match your experience with the particular feature or requires further clarification, please use this form to report a documentation issue.