What is difference between SingleEmailMessage and MassEmailMessage in Salesforce?

SingleEmailMessage-The SingleEmailMessage can send message related to only single record.
setTargetObjectId – The ID of the contact, lead, or user to which the email will be sent.
Used for single record
Messaging.SingleEmailMessage

MassEmailMessage -MassEmailMessage can send mails related to multiple records (WhatId and TargetObjectId).
setTargetObjectIds – A list of IDs of the contacts, leads, or users to which the email will be sent.
Used for multiple records
Messaging.MassMailMessage.

Hi,

Single emails are like regular individual emails that may go to one or more addresses (to/cc/bcc), but each of these emails has the same body.
Mass emails typically go to a large number of addresses (currently capped to 250 per email), with personalized message bodies.
If you wanna send Mass Email then take a look at the salesforce app Massmailer.
MassMailer lets you send unlimited emails from within Salesforce, no matter which edition you use. Built on the Salesforce platform, it has all the mass email features you’ll ever need.Send unlimited emails no matter how large your email list isNative Salesforce integration for a seamless experience
Beautiful email templates complete with HTML and text versions
Simple email creation wizard for point-and-click ease
Detailed metrics & monitoring so you’re always in the know
Email verification & deliverability tools to make sure your emails reach your prospects

Hope this helps.