"Chaining" refers to a Workflow where each form is visited in order. That is, each Link joins one form to another. Since each form has just the single entry point, you can be sure that users will progress through the Workflow in a defined order.
"Grouping" refers to a Workflow where the initial form links to multiple forms. Due to this, each of the subsequent forms can be filled out in any order. This can be helpful when you want to prompt for follow-up information and/or to compile a "packet" of documents from a user. When using this method, the user would not need all of their information at once, and each sub-forms' results can go to different recipients.