Explain the concept of window functions in database management systems and their application.

Window functions in DBMS allow performing calculations across a set of table rows that are related to the current row, without grouping the data. They extend the capabilities of aggregate functions by providing results for each row while preserving the other data.

Main features:

• Operate over a window of rows defined by OVER().
• Can specify order (ORDER BY) and partition (PARTITION BY).
• Enable calculations like moving sums, ranking, cumulative totals, etc.

Example in SQL:

SELECT
  employee_id,
  department_id,
  salary,
  RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS salary_rank
FROM employees;

Here, each employee's salary rank within their department is calculated.

In C#, window functions are usually applied via LINQ or through SQL queries to a database that supports window functions.