A bit is the smallest possible unit of information. A bit holds one of two states and so conveys either yes/no, up/down, left/right, true/false, A or B…any distinction, as long as there are just two possibilities. The distinction represented by a bit ultimately depends upon agreement between whoever sets the bit and whoever reads it. A mailbox flag, for example, is a bit: it holds one of two states, up or down, and by pre-arrangement, up indicates ‘mail is inside’ while down indicates ‘no mail’; if the flag gets stuck in a position half-way between up and down, its meaning becomes indeterminate.
Computers store bits in several different ways, most commonly as electronic circuits, as charges on magnetic surfaces, or as microscopic grooves on optical discs. Whatever the physical mechanism, we usually represent the two states of a bit with the symbols 0 and 1. Be clear, though, that the choice of symbols is really arbitrary, and bits aren’t always used to represent numbers. In principle, bits can be used to represent any kind of data, including text, audio, images…anything.
While a single bit can only represent two states, for every additional bit, we can represent twice as many states. With three bits, for example, we can represent
2 * 2 * 2 = 2^3 = 8 states (
* means multiplication and
^ means exponentiation). With three bits, we have these eight possible combinations of bits, each of which can arbitrarily represent whatever we want, like say different colors:
000 blue 001 red 010 orange 011 yellow 100 green 101 purple 110 white 111 black
The general formula is: given n bits, we can represent 2^n different values
For example, given 7 bits, we can represent
2^7 = 128 different values.
These prefixes can all be used with –bit as well as –byte, e.g. a mebibit is 2^20 bits.
In abbreviations, uppercase B usually means byte while lowercase b usually means bit, e.g. KB is a kilobyte while kb is a kilobit. Usually the case of the prefix letter matches the case of the b, but these conventions are not always strictly followed!
When talking about throughput, the convention is to use bits, not bytes, e.g. the download speed of my internet connection is 100mbps (100 megabits per second).