What is the difference between bit rate and baud rate?

While bit rate and baud rate are closely linked, they aren't the same thing. Bit rate is a measure of the number of data bits -- zeros and ones -- that can be transmitted in one second. A bit rate of 2,400 bits per second (bps) for example, would mean 2,400 zeros and ones are transmitted each second.

Baud rate represents the number of times per second a signal (changing from zero to one or one to zero) or symbol (the connection's voltage, frequency or phase) in a communications channel changes state or varies. For example, a 2,400 baud rate means the channel is changing states up to 2,400 times per second. In this case, the baud rate is the same as the bit rate: 2,400 bps, and a bit rate of one equals a baud rate of one.

While bit rate and baud rate have similarities, they aren't the same.

analog telephone modems were simpler than digital communications, and often bits per second and baud were the same, with one symbol (baud) transmitting one bit. To make communications faster, more advanced modem use modulation techniques that send more bits per symbol. As a result, bit and baud rates aren't always the same number.

Depending upon the modulation used, if a channel can send up to four bits per baud, the baud state would change just once. So, the baud rate would be one-fourth the bit rate in this case.

You can think of the baud rate formula like this: bits per second = baud per second x the number of bits per baud.