Here are illustrations of the canned paragraph formats listed in the Paragraph dropdown of WordPress’ builtin WYSIWYG editor (in Visual mode):

This is the default Paragraph format.

This is Preformatted.

This is Heading 1.

This is Heading 2.

This is Heading 3.

This is Heading 4.

This is Heading 5.

This is Heading 6.

This is the default Paragraph format (again). Using the default Twenty Eleven theme, it appears that only some of these paragraph formats actually look different from each other.