You can access the system clipboard, which allows passing and receiving values from other processes, by the functions clipboard and set-clipboard. These can deal with strings and images, and metafiles on Cocoa and Microsoft Windows. When used inside the same Lisp process, they can also be used to pass Lisp values. Use clipboard-empty to check if there is anything in the clipboard. See also Edit actions on the active element.
Similarly, the primary selection of the GUI system can accessed by the function selection, set-selection and selection-empty.
CAPI User Guide and Reference Manual (Macintosh version) - 25 Feb 2015