A bit cell is the amount of space on a digital storage medium used to host a single bit of information. Bits are the smallest units utilized by a computer, and are written in binary, represented as either a "1" or a "0". These cells act as a type of density measurement, indicating how thickly packed data can be compressed on a specific storage medium. Under normal circumstances — with all other things being equal — a storage medium with a smaller bit cell rate will be more efficient than one that cannot store as many bits in the same area of space.
The concept of bit cells applies to all types of storage mediums, including electronic and magnetic storage, such as hard and flash drives. No matter the storage type, the concept itself never changes; it always refers to either a specific measurable area within which a bit can be stored. For traditional magnetic storage devices, such as hard drives and magnetic tape, a bit rate is expressed in physical units of space: one bit per millimeter of storage tape or one bit per square millimeter for magnetic storage platters. When considering purely electronic storage devices such as a flash drive, it is expressed in terms of circuits, such as the number of bits that can be stored in a single circuit within the device.
Bit cells become important when comparing the density of storage mediums. In computing, smaller is often better, both from an efficiency and space-management standpoints. Storage mediums that provide a higher density of bits — in other words, a smaller bit cell rate — can compact more information into a smaller amount of space.
The benefits to this are simple to grasp; a device with half the bit cell rating of another can store twice as much information using the exact same amount of space. One example of favoring storage mediums with smaller rating of bit cells is illustrated by the widespread move from digital tape reels to hard drives. As hard drives generally provide a smaller bit cell rate, they can fit more easily into home computing cases, providing affordable storage without taking up an excessive amount of real estate within the machine.
Although bit cell rating is important, it is not the final word when it comes to storage medium performance. Other factors such as the speed of the drive, the method by which it interfaces with the computer, and the storage medium type all combine to determine the overall performance of the hardware. Taking bit cell rating into account is only one piece in the puzzle to create efficient, high-performing, and reliable storage devices.