This is possible through a workflow and an email alert action. However, if you want to use a trigger, you can use a before update trigger on the opportunity.
trigger OpportunityTrigger on Opportunity(before insert, before update){ //Code to Send the email. }
Here is how you can send an email. http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_forcecom_email_outbound.htm
However, if you want to use a trigger, you can use a before update trigger on the opportunity.
trigger OpportunityTrigger on Opportunity(before insert, before update){
//Code to Send the email.
}
Here is how you can send an email.
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_forcecom_email_outbound.htm
Regards,
Satish Kumar
Can you give me the code for trigger..how to send a mail