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On multi-window terminals, the most recent kill done in Emacs is also the primary selection, if it is more recent than any selection you made in another program. This means that the paste commands of other applications with separate windows copy the text that you killed in Emacs. In addition, Emacs yank commands treat other applications' selections as part of the kill ring, so you can yank them into Emacs.
Many window systems follow the convention that insertion while text is selected deletes the selected text. You can make Emacs behave this way by enabling Delete Selection mode, with M-x delete-selection-mode, or using Custom. Another effect of this mode is that <DEL>, C-d and some other keys, when a selection exists, will kill the whole selection. It also enables Transient Mark mode (see Transient Mark).