public class testemail
{
private final Contact con;
public testemail(ApexPages.StandardController controller)
{
this.con=(Contact)controller.getRecord();
}

public void SendEmail()
{
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
mail.setTargetObjectId(con.Id);
mail.setTemplateId('00X90000000QHUD');
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
}
}

<apex:page standardcontroller="Contact" extensions="testemail">
<apex:form>
<apex:commandButton value="Send Email!" action="{!SendEmail}"/>
</apex:form>
</apex:page>

emailtemplatelist = new List<EmailTemplate>();
for ( EmailTemplate e : [select Id,Name,Subject,body from EmailTemplate where name like :userinputtemplatename+'%'])
{
emailtemplatelist.add(e);
}  

If you do not want to use a template you can specify the toAddress, Subject and Body of the email using the appropriate methods. Below is a piece of code which sends an email not associated to an email template.

 String[] toaddress = new String[]{};
toaddress.add(emailaddress[i]);
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
mail.setToAddresses(toaddress);
mail.setsubject(subject);

There may and will arise situations in which you have a Email Template.. You would not have any merge fields in this Email Template. ..

You would like to send an Email using this Email Template.. But you do not have a Contact, or a Lead or a User to set as the targtObjectId...

In such cases, you can create a Contact on the fly, send the email and then delete the Contact... Remember to create the contact with Name, and Email since these are mandatory fields to send an Email...

Now that you have learnt to use the single email message, the mass email message is much simpler to understand...

You just need to replace the targetobjectId with setTargetObjectIds... And of course, you will have to specify a list of valid Contact,User or Lead ID's...

For the same example, just replace the Apex code as below...

public class testemail
{
  private final List<Id> contactids;
  public List<Contact> con;
  public testemail(ApexPages.StandardController controller)
  {
     con = [select Id from Contact limit 250 ];
     for(Integer i=0;i<250;i++)
     {
         contactids.add(con[i].Id);
      }  
  }

  public void SendEmail()
  {
       Messaging.MassEmailMessage mail = new Messaging.MassEmailMessage();
       mail.setTargetObjectIds(contactids);
       mail.setTemplateId('00X90000000QHUD');
       Messaging.sendEmail(new Messaging.MassEmailMessage[] { mail });
  }  


}

What we do is, query the contact object and get a list of Contact Id's... We then set this list of ID's as the targetobjectId's....

Relationship between objects are created when you create a field of type Look-up... Of course, Salesforce standard objects come with their own built-in relationships.. Let's categorize relationships into two categories...

• Standard Relationship
• Custom Relationship

We all know how a relationship looks like in the UI, for ex: Account - Contact relationship. You can see that one Account can have many Contacts associated to it . When you create or edit a Contact you will have to select a Account from the look-up dialog to establish a relationship...

Now, that's all what we know.. But at the database level there is much more to be explored..

Standard Relationship

Standard relationships are the one's that come pre-built between Standard objects in Salesforce.

Let's understand the Account-Contact relationship.. In Contact you can find a field called Account.. This field establishes the relationship.. When you view a Contact record you will see the Account Name populated in this field... But guess what, that is not actually stored in the database..

The ID of the Account is stored in the look-up field. Hence, all look-up fields would store the ID.

Here is how the actual database fields are....

AccountId - This stores the ID of the Account.
Account -- This stores the relationship of the Account.

So, here come's the interesting part. By using the "Account" relationship you can access any field of the associated Account.

[Select Account.Name,Account.BillingCity from Contact]

And what more, you can retrieve data from a relationship to a relationship as well...

[Select Account.Name,Account.BillingCity,Account.Owner.Name from Contact]

So, they can be multi-level....

What's the advantage??

The better you understand relationships, the better you can construct complex nested queries and try to minimize the number of queries used.

Custom Relationship:

This is same as the standard relationship except that the naming convention has some differences...

THE __c and __r :

In our same example, if we create a custom Account look-up field on Contact and name it as CustomAccount... then accessing the relationship is as follows...

CustomAccount__c --> Equivalent of AccountId, holds the ID of the Account.

CustomAccount__r --> Equivalent of Account, holds the relationship

Our example query will modify as below..

[Select CustomAccount__r.Name,CustomAccount__r.BillingCity,CustomAccount__r.Owner.Name from Contact]

Wondering how i came to know about this and where will you find more details on this????

Go to Setup--> Develop --> API and click on "Generate Enterprise WSDL"

The WSDL shows the actual way data is stored, the actual field names, the actual relationships etc...