Constant Bit Rate - This is best used from a streaming server NOT to be confused with Progressive Download (http). On streaming servers one would need to have very tight control over the bandwidth in use at any moment. CBR does that.
With CBR, the encoder decides what data to "throw out" in order to keep the constant bit rate. Complex video such as dissolves, camera motion, fast action suffers most as far as image quality, whereas "talking head" video would suffer less loss in quality. Complex video needs more data to look good.