hi gyus... This code is working fine..when ever I insert or update record it sends email.. but I am quite confused  so plz explain me the use of line 16,line 19,line 25 & what is the use of flag... and tell me that is messaging.singleEmailMessage is defined keyword...??
Last but not the least I want to edit the code in such a way that if i insert the data with dataloader where there is more than one contact then send email to more than one contact is inserted at a time....

************apex class**********

public with sharing class HelperContactTrigger {
    public static List<Contact> sendEmail(List<Contact>Contacts)
    {
     //query on template object
        EmailTemplate et=[Select id from EmailTemplate where name=:'Sales: New Customer Email'];

        //list of emails
        List<Messaging.SingleEmailMessage> emails = new List<Messaging.SingleEmailMessage>();   
        
        for(Contact con : Contacts)
        {
          //check for Account
            if(con.AccountId != null && con.Email != null){

                //initiallize messaging method
                Messaging.SingleEmailMessage singleMail = new Messaging.SingleEmailMessage();

                //set object Id
                singleMail.setTargetObjectId(con.Id);

                //set template Id
                singleMail.setTemplateId(et.Id);

                //flag to false to stop inserting activity history
                singleMail.setSaveAsActivity(false);

                //add mail
                emails.add(singleMail);
            }
        }
            //send mail
        Messaging.sendEmail(emails);

        return Contacts;          
        
    }

    public static List<Contact> sendEmailafter(List<Contact>Contacts)
    {
    //query on template object
        EmailTemplate et=[Select id from EmailTemplate where name=:'Sales: New Customer Email'];

        //list of emails
        List<Messaging.SingleEmailMessage> emails = new List<Messaging.SingleEmailMessage>();   
        
        for(Contact con : Contacts)
        {
          //check for Account
            if(con.AccountId != null && con.Email != null){

                //initiallize messaging method
                Messaging.SingleEmailMessage singleMail = new Messaging.SingleEmailMessage();

                //set object Id
                singleMail.setTargetObjectId(con.Id);

                //set template Id
                singleMail.setTemplateId(et.Id);

                //flag to false to stop inserting activity history
                singleMail.setSaveAsActivity(false);

                //add mail
                emails.add(singleMail);
            }
         }
            //send mail
        Messaging.sendEmail(emails);

        return Contacts;          
        }
    }

*********Apex trigger********

trigger SendEmailToAccount on Contact (after insert,after update) 
{
    if(Trigger.isAfter)
    {
        if(Trigger.isInsert )
        { 
            //helper class for single email but bulk messages
            HelperContactTrigger.sendEmail(trigger.new);
        }
    }
        if(trigger.isAfter && trigger.isUpdate )
        {           
         HelperContactTrigger.sendEmailafter(trigger.new);
        }
}