In most graphical desktop environments, you can transfer data (usually text) between different applications using a system facility called the clipboard. On X, two other similar facilities are available: the primary selection and the secondary selection. When Emacs is run on a graphical display, its kill and yank commands integrate with these facilities, so that you can easily transfer text between Emacs and other graphical applications.
By default, Emacs uses UTF-8 as the coding system for inter-program
text transfers. If you find that the pasted text is not what you
expected, you can specify another coding system by typing C-x
RET x or C-x RET X. You can also request a
different data type by customizing
See Coding Systems for Interprocess Communication.