The capacity of a device to hold and retain data, it usually refers to auxiliary storage (or "mass storage"). There are a variety of techniques and devices for storing large amounts of data (for example, disk drives or tape drives). Optical disks are a preferred method because they use a laser to read and write data and have very large storage capacities. Data storage is measured in kilobytes (1,024 bytes), megabytes (1,024 kilobytes), gigabytes (1,024 megabytes), and terabytes (1,024 gigabytes). Data storage is not to be confused with "memory," which refers to temporary storage areas within a computer.