SingleEmailMessage

MassMailMessage

The SingleEmailMessage can send message related to only single record.

MassEmailMessage can send mails related to multiple records (WhatId
and TargetObjectId).

setTargetObjectId – The ID
of the contact, lead, or user to which the email will be sent.

setTargetObjectIds – A
list of IDs of the contacts, leads, or users to which the email will be sent.

Used for single record

Used for multiple records

Messaging.SingleEmailMessage

Messaging.MassMailMessage