How can Use LINQ in C#

Language Integrated Query (LINQ) adds the ability to query objects using .NET languages. The LINQ to SQL object/relational mapping framework provides the following basic features:
LINQ has a power of querying on any source of data, data source could be the collections of objects, database or XML files. We can easily retrieve data from any object that implements the IEnumerable interface. Microsoft basically divides LINQ into three areas and that are give below.

•LINQ to Object {Queries performed against the in-memory data}
•LINQ to ADO.Net
◦LINQ to SQL (formerly DLinq) {Queries performed against the relation database only Microsoft SQL Server Supported}
◦LINQ to DataSet {Supports queries by using ADO.NET data sets and data tables}
◦LINQ to Entities {Microsoft ORM solution}
•LINQ to XML (formerly XLinq) { Queries performed against the XML source}
Tools to create classes (usually called entities) mapped to database tables
Compatibility with LINQ’s standard query operations
The DataContext class, with features such as entity record monitoring, automatic SQL statement generation, record concurrency detection

Code for LINQ**************************

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using pmsContract.entities;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;
using System.Data.Linq;
using System.Data.Linq.Mapping;

namespace pms.Webproject
{
//we have create class usertest and table nam: pms_user

[Table(Name = "pms_user")]//define table name
public class usertest //this is classs
{
[Column]
public string userfname { get; set; } //get or set table fields
[Column]
public string userlname { get; set; }
[Column]
public string useremailID { get; set; }

}

public partial class test : System.Web.UI.Page
{

protected void Page_Load(object sender, EventArgs e)
{
string conn = ConfigurationManager.ConnectionStrings["stringConnection"].ToString();
DataContext db = new DataContext(conn);//Interface between linq or SQL
Table<usertest> pmsusettest = db.GetTable<usertest>();//create object of table
var query = from d in pmsusettest orderby d.userfname select d; //select linq query for data selection.
GridView1.DataSource = query;//provide datasource to gridview
GridView1.DataBind();
}

}
}