Why do we always prefer using parameters in SQL statements?

I am very new to working with databases. Now I can write SELECT, UPDATE, DELETE, and INSERT commands. But I have seen many forums where we prefer to write:

SELECT empSalary from employee where salary = @salary

...instead of:

SELECT empSalary from employee where salary = txtSalary.Text

Why do we always prefer to use parameters and how would I use them?

I wanted to know the use and benefits of the first method. I have even heard of SQL injection but I don't fully understand it. I don't even know if SQL injection is related to my question.

Solution

Using parameters helps prevent SQL Injection attacks when the database is used in conjunction with a program interface such as a desktop program or web site.

In your example, a user can directly run SQL code on your database by crafting statements in txtSalary.

For example, if they were to write 0 OR 1=1, the executed SQL would be

 SELECT empSalary from employee where salary = 0 or 1=1

whereby all empSalaries would be returned.

Further, a user could perform far worse commands against your database, including deleting it If they wrote 0; Drop Table employee:

SELECT empSalary from employee where salary = 0; Drop Table employee

The table employee would then be deleted.

In your case, it looks like you're using .NET. Using parameters is as easy as:

string sql = "SELECT empSalary from employee where salary = @salary";

using (SqlConnection connection = new SqlConnection(/* connection info */))
using (SqlCommand command = new SqlCommand(sql, connection))
{
    var salaryParam = new SqlParameter("salary", SqlDbType.Money);
    salaryParam.Value = txtMoney.Text;

    command.Parameters.Add(salaryParam);
    var results = command.ExecuteReader();
}

Dim sql As String = "SELECT empSalary from employee where salary = @salary"
Using connection As New SqlConnection("connectionString")
    Using command As New SqlCommand(sql, connection)
        Dim salaryParam = New SqlParameter("salary", SqlDbType.Money)
        salaryParam.Value = txtMoney.Text

        command.Parameters.Add(salaryParam)

        Dim results = command.ExecuteReader()
    End Using
End Using

Edit 2016-4-25:

As per George Stocker's comment, I changed the sample code to not use AddWithValue. Also, it is generally recommended that you wrap IDisposables in using statements.