Grouping operators generates a categories or groups of an item on the basis of given key. These type of operators implements IEnumerable collection of type IGrouping<Key, SourceItem> where Key is a key value, on the basis of which grouping is being done and SourceItem is the collection of items.

In linq, We have a two types of grouping operators:
1. GroupBy
2. ToLookup

1. GroupBy:- It takes a collection of items and returns  a group of items from the given collection based on some key value.

Query syntax :-
Lets take an example we have a collection of an employees of a different departments. We need to make a groups of an employee of a same department.

Code:-

IList<Employee> empList = new List<Employee>() { 
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "John", Dept = "Electronic Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Steve",  Dept = "Computer Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(),  EmpName = "Bill",  Dept = "Geographical Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Ram" , Dept = "Computer Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(),  EmpName = "Abram" , Dept = "Geographical Department" } 
    };
var result = from e in empList 
                    group e by e.Dept;
foreach (var deptGroup in result)
{
    Console.WriteLine("Department Group: {0}", deptGroup .Key); 
    foreach(Employee e in deptGroup ) 
     {
           Console.WriteLine("Employee Name: {0}", e.EmpName);
      }
}

Output:-

Method syntax for above code:-

IList<Employee> empList = new List<Employee>() {
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "John", Dept = "Electronic Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Steve",  Dept = "Computer Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Bill",  Dept = "Geographical Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Ram" , Dept = "Computer Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Abram" , Dept = "Geographical Department" }
    };
         var result = empList .GroupBy(e => e.Dept);
            foreach (var deptGroup in result)
            {
                Console.WriteLine("Department Group: {0}", deptGroup.Key);
                foreach (Employee e in deptGroup)
                {
                    Console.WriteLine("Employee Name: {0}", e.EmpName);
                }
            }

2.  ToLookup:-
ToLookup operator is similar to a GroupBy operator but their execution way is different because GroupBy operator executes delay while ToLookup operator executes instantly. ToLookup also not supports a query syntax, it supports only a method syntax.

Example:-
Let's take an example, we have a collection of employees of a different departments. We need to make a group of an employee of a same department.

Code:-

IList<Employee> empList = new List<Employee>() {
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "John", Dept = "Electronic Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Steve",  Dept = "Computer Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Bill",  Dept = "Geographical Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Ram" , Dept = "Computer Department" } ,
        new Employee() { EmpID = Guid.NewGuid().ToString(), EmpName = "Abram" , Dept = "Geographical Department" }
    };
         var result = empList .ToLookup(e=>e.Dept);
            foreach (var deptGroup in result)
            {
                Console.WriteLine("Department Group: {0}", deptGroup.Key);
                foreach (Employee e in deptGroup)
                {
                    Console.WriteLine("Employee Name: {0}", e.EmpName);
                }
            }

Output:-