Structured Query Language (SQL) is a special-purpose programming language designed for managing data held in a Relational Database Management System (RDBMS). SQL-like languages can also be used in Relational Data Stream Management Systems (RDSMS), or in "not-only SQL" (NoSQL) databases.
SQL comprises of 3 major sub-languages:
The core DML operations are Create, Read, Update and Delete (CRUD for short) which are performed by the statements
There is also a (recently added)
MERGE statement which can perform all 3 write operations (INSERT, UPDATE, DELETE).
Many SQL databases are implemented as client/server systems; the term "SQL server" describes such a database.
At the same time, Microsoft makes a database that is named "SQL Server". While that database speaks a dialect of SQL, information specific to that database is not on topic in this tag but belongs into the SQL Server documentation.