Most commands which erase text from the buffer save it in 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.
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.