ExecuteReader - This method returns a DataReader which is
filled with the data that is retrieved using the command
object. 

ExecuteNonQuery - This method returns no data at all. It is
used majorly with Inserts and Updates of tables. 

ExecuteScalar - Returns only one value after execution of
the query. It returns the first field in the first row. 

This would be excellent for receiving a count of records
(Select Count(*)) in an sql statement, or for any query
where only one specific field in one column is required.

Execute Reader: it executes the command and returns the result 
set of the command

Execute NonQuery: it executes the command and returns the
number of rows affected

Execute Scalar: it executes the command and returns only
first row first column value

ExecuteReader-It is readonly forward only.It executes the 
command and return the reader objectl.It executes only 
select command.

ExecuteNonQuery-It executes command and return's no 
value.It returns only how many row effected.It returns 
value when the commandType is stroedProcedure with output 
parameter.

ExecuteScalar-It executes the command and return a single 
value.That is 1st column and 1st row.