What is difference between Vim's clipboard "unnamed" and "unnamedplus" settings?

On Mac OS X and Windows, the * and + registers both point to the system clipboard so unnamed and unnamedplus have the same effect: the unnamed register is synchronized with the system clipboard.

On Linux, you have essentially two clipboards: one is pretty much the same as in the other OSes (CtrlC and CtrlV in other programs, mapped to register + in Vim), the other is the "selection" clipboard (mapped to register * in Vim).

Using only unnamedplus on Linux, Windows and Mac OS X allows you to:

• CtrlC in other programs and put in Vim with p on all three platforms,
• yank in Vim with y and CtrlV in other programs on all three platforms.

If you also want to use Linux's "selection" clipboard, you will also need unnamed.

Here is a cross-platform value:

set clipboard^=unnamed,unnamedplus

Reference:

:h 'clipboard'
(and follow the tags)