Memory cells were a part of Data's positronic brain forming the memory record.
Believing he perished in a shuttlecraft explosion, Wesley Crusher was examining Data's medals and awards which he noted as some of Starfleet's highest honors. Geordi La Forge responded "Not bad for a walking pile of circuitry and memory cells." (TNG: "The Most Toys")
The contents of the journals and logs of the colonists of Omicron Theta were transferred into Data's memory cells. Furthermore, Doctor Noonien Soong, Data's creator, also experimented with scanning the synaptic patterns of the colonists' temporal lobes and programmed them into his neural nets. (TNG: "Silicon Avatar")
In 2369, Data kept a full lecture of Wagner in his memory record. (TNG: "Timescape")