In digital communication, the data is often generated in digital pulses.

But for Tx, the data codes have to be transmitted over analog medium.

The transmission over transmission lines are called as line codes.

These line codes can be categorized as RZ and NRZ as well as

Uni-polar and bipolar.

RZ & NRZ : RZ stands for return to zero, while NRZ stands for Non RZ

It means that the square pulse has only positive or negative levels pt does not have zero voltage levels.

Uni-polar and bipolar : Uni-polar means that the resulting wave will fluctuate only between zero voltage and positive level. While bipolar means voltage can fluctuate, in both positive and negative levels.