Nonclustered indexes are stored separately from the table. Each index in this structure contains a pointer to the row in the table which it represents.
This pointers are called a row locators. The structure of the row locator depends on whether the data pages are stored in a heap or a clustered table. For a heap, a row locator is a pointer to the row. For a clustered table, the row locator is the clustered index key.
An example of creating a non clustered index on table Employees and column Employee_Surname:
CREATE NONCLUSTERED INDEX ix_employees_name ON Employees(Employee_Surname);
There can be multiple nonclustered indexes on the table. The read operations are generally slower with non clustered indexes than with clustered indexes as you have to go first to index and than to the table. There are no restrictions in write operations however.