From LQWiki

Core memory was a precursor to today's Dynamic Random Access Memory RAM. Core memory consisted of arrays of doughnuts made of magnetic ferrite material, hand strung with wire. Since it had to be hand-strung, core memory was too expensive to use until manufactoring of it moved to third-world Asian countries. This allowed it to displace the older drum memory.

Unlike present day dynamic RAM, core memory was static - it didn't lose data when it was powered down. Core memory "remembered" its state until it was written to. Core memory also allowed random access. The older drum memory, which was similar to modern day hard drives, had to wait for the needed data to pass under the heads. (In the Story of Mel, Mel used this trait of drum memory to get rid of the neccessity of a delay loop.)

Core memory, while obsolete, lives on in modern day computer lingo. The most common example of this is core dump.