The logic behind having an OK button and Apply button

Moving back to desktop land after being drunk with mobile interfaces, desktop dialogs almost always have these three buttons:

• OK
• Apply
• Cancel
• A close button.

This pattern seems to occur on both Windows and Linux-based UIs. Never tried other OSes. May have existed in Macs, but maybe I just can't remember.

It never occurred to me before that they were redundant until just now. The Cancel button is the same as the close button. Why was it placed there in the first place if both things do the same? They both close the dialog.

Also applies to the OK and Apply buttons as well. I tend to click either. Both do the same thing. Sure, Apply will apply changes without closing the dialog. But most of the time, you have already set all the preferences you needed, and just needs the OK to confirm.

• What was the logic for such design?
• Why a Close when there's a Cancel?
• What makes Apply differ from OK?