Canned Responses can be used to quickly insert pre-formatted reply templates into your messages. You can make your pre-formatted replies even more personal, by inserting the requester's name, agent signature and ticket details using dynamic placeholders. You can choose to create personal canned responses or have your Admin create some for you.
Whereas for requests which come in with limited information, the agents might end up asking a set of repetitive questions to the customers. While replying, the customer would have to copy and paste the set of questions before they begin to answer them. Using Canned forms, you can create forms that can be filled in by users to provide the necessary information. An Admin will have to create the canned forms for the helpdesk.