Most commands which erase text from the buffer save it in the kill
ring (see The Kill Ring). These are known as kill commands, and
their names normally contain the word ‘kill’ (e.g.,
kill-line). The kill ring stores several recent kills, not
just the last one, so killing is a very safe operation: you don’t have
to worry much about losing text that you previously killed. The kill
ring is shared by all buffers, so text that is killed in one buffer
can be yanked into another buffer.
When you use C-/ (undo) to undo a kill command
(see Undo), that brings the killed text back into the buffer, but
does not remove it from the kill ring.
On graphical displays, killing text also copies it to the system clipboard. See “Cut and Paste” Operations on Graphical Displays.
Commands that erase text but do not save it in the kill ring are
known as delete commands; their names usually contain the word
‘delete’. These include C-d (delete-char) and
DEL (delete-backward-char), which delete only one
character at a time, and those commands that delete only spaces or
newlines. Commands that can erase significant amounts of nontrivial
data generally do a kill operation instead.
You can also use the mouse to kill and yank. See “Cut and Paste” Operations on Graphical Displays.