The kill ring is a list of blocks of text that were previously killed. There is only one kill ring, shared by all buffers, so you can kill text in one buffer and yank it in another buffer. This is the usual way to move text from one buffer to another. (There are several other methods: for instance, you could store the text in a register; see Registers. See Accumulating Text, for some other ways to move text around.)
The maximum number of entries in the kill ring is controlled by the variable kill-ring-max. The default is 60. If you make a new kill when this limit has been reached, Emacs makes room by deleting the oldest entry in the kill ring.
The actual contents of the kill ring are stored in a variable named kill-ring; you can view the entire contents of the kill ring with C-h v kill-ring.