When it comes to stored procedures and functions, PostgreSQL differs quite significantly from other database systems. Most database engines force you to use a certain programming language to write server-side code. Microsoft SQL Server offers Transact-SQL, while Oracle encourages you to use PL/SQL. PostgreSQL doesn't force you to use a certain language; instead, it allows you to decide on what you know best and what you like best.
The reason PostgreSQL is so flexible is actually quite interesting in a historical sense, too. Many years ago, one of the most well-known PostgreSQL developers, Jan Wieck, who had written countless patches back in its early days, came up with the idea of using TCL as the server-side programming language. The trouble was that nobody wanted to use TCL and nobody wanted to have this stuff in the database engine...
