Constant Bitrate, or CBR, refers to video or audio encoding where the bitrate used doesn't fluctuate. While CBR audio is actually rather common, being used for such technologies as CD-A (standard audio CDs), Dolby Digital, and even many MP3 files. For video its less common, as Variable Bitrate (VBR) encoding generally offers far greater quality at a comparable bitrate.
Where CBR Is Used
There are several legitimate uses for CBR encoding. The most obvious is hardware based encoding. Unlike software video encoders, most consumer hardware encoders are limited to encoding in a single, realtime pass. Since 2 pass encoding allows software to make its final decisions based on information about the entire video stream, it can allocate the bitrate in such a way to get optimal quality. Hardware that only encodes in realtime must make the same decisions based on only small amounts of the video, therefore either CBR encoding or VBR with little variation is generally the only way to get high quality results.
Some formats, such as DV, use a standard bitrate across all devices and software. This is part of what makes DV a reasonably good editing format. It also makes the size of DV video predictable based on nothing but length. The same approach is standard for professional editing formats, although the bitrates tend to be significantly higher than for DV. Uncompressed RGB video is always encoded CBR, but has such a high bitrate that it's generally not used, especially since it must be converted to the more compact YUV video format before encoding to any consumer format.
The size of any stream (video, audio, or otherwise) can be calculated by multiplying the bitrate its encoded at by its length. When calculating filesize based on bitrate you also have to take into account any overhead introduced by the Container being used, such as AVI or MPEG. Container overhead varies from one to another, and can even be affected by the number or type of streams being muxed.
Although most computer oriented applications measure bitrate in terms of binary kilobits and Megabits where 1kb (1 kilobit) = 1024 bits and 1Mb (1 Megabit) = 1024 kilobits, bitrate calculations use standard metric values for these prefixes, meaning 1kb = 1000b and 1Mb = 1000kb. Bitrate calculations are typically made in kilobits per second (kbps) or Megabits per second (Mbps). They can also be expressed in other units, including bits per second (bps) or even Megabytes per second (MBps). Notice the capital B denoting Bytes. You should always use a lower case b to denote bits and upper case for Bytes.