SQL is an acronym for Structure Query Language, it is a language which helps program to communicate with the databases, regardless of the type of the database, the SQL syntax is almost the same except for extended commands which certain DBMS support or do not support or simply have its own versions or dialects.
Now what does DBMS mean, DBMS stands for database Management System which receives the query and then returns the appropriate set of result for that query.
Unlike other computing languages, SQL syntax is quite easy to understand. SQL is responsible for inserting, updating, reading, and deleting of data. SQL also provides you other features beside inserting, updating or deleting of data, like creating views, tables, stored procedures, and setting read/write permissions on tables for some users.
SQL in short is an interface or a channel with which a program can communicate with the database, i.e. your website written in any server side language like PHP, ASP, JSP or other desktop applications written in java, c/c++ and other platforms can access data from the database by using the SQL queries, which are always the same no matter whatever language is used to develop the front end of the software/website.