Motor Encoders and Roller Encoders

We have a plate making machine which has two identical roller sets, at the input & output side respectively, the plate passes through these pair of rollers & encoders are mounted at each end of the roller, the other end has the drive motor. This is to ensure consistent movement of the plate .

Please let me know the correct working of a motor encoder & a roller encoder...do they work in a similar fashion? Secondly, when we specify the encoder as say 2000 segment encoder, what exactly this numeric figure means?