If you look at any professionally typeset book or magazine, you will see that they do not use straight-up-and-down quote marks (") or apostrophes ('). Instead, their quotes and apostrophes have a curved shape. The curved ones are called typographically correct quotes, because they are the correct ones to use when doing typesetting.
At small sizes, it’s hard to see which one is being used, but at larger sizes, it’s really obvious:
| Quote Mark | Description | HTML to create it |
|---|---|---|
| ' | Simple apostrophe | ' |
| ’ | Typographically correct apostrophe | ’ |
| " | Simple double quote | " |
| “ | Typographically correct left double quote | “ |
| ” | Typographically correct right double quote | ” |