What is the main difference between a block letter style and a modified block letter style?

A full block style business letter is all justified against the left margin, including the your return address (if not using letterhead), the reference line, and your closing, signature, and printed name.

A modified block style business letter is justified against the left margin with the exception of including the your return address, the reference line, and your closing, signature, and printed name. These are tabbed about one third to the right of the page.