trigger NotifyTaskOwner on Task (After Update) {

    List<Id> taskIdList;    
    Map<Id, List<Id>> ownerIdTaskIdMap = new Map<Id, List<Id>>();

    Task oldTask;
    for(Task newTask : Trigger.new){
        oldTask = Trigger.oldMap.get(newTask.id);
        
        if(newTask.status != oldtask.status  || newTask.Description != oldTask.Description) {
            taskIdList = ownerIdTaskIdMap.get(newTask.ownerId);
            
            if(taskIdList == null) {
                taskIdList = new List<Id>();
            }
            
            taskIdList.add(newTask.Id);
            ownerIdTaskIdMap.put(newTask.ownerId, taskIdList);
        }
    }
    
    if(!ownerIdTaskIdMap.isEmpty()) {
		Map<Id, User> userMap = new Map<Id, user>([select id, email from user where id in: ownerIdTaskIdMap.keySet()]);
        
        //list of emails
        List<Messaging.SingleEmailMessage> emails = new List<Messaging.SingleEmailMessage>();

        String userEmail, emailBody;
        String emailSubject = 'Task updated'; // define your own subject
        
        //loop
        for(Id ownerId : ownerIdtaskIdMap.keySet()){
			userEmail = userMap.get(ownerId).Email;
            taskIdList = ownerIdtaskIdMap.get(ownerId);
            emailBody = 'Email body ' + taskIdList;
            
            for(Id tskId : taskIdList){
                // compose your email body content
            }
            
            Messaging.SingleEmailMessage singleMail = new Messaging.SingleEmailMessage();	//initiallize messaging method
            singleMail.setToAddresses(new String[] {userEmail}); 
            singleMail.setSubject(emailSubject); 
			singleMail.setPlainTextBody(emailBody); 
	        emails.add(singleMail);	//add mail
        }

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