Non-return-to-zero (NRZ), non-return-to-zero-inverted (NRZI), return-to-zero (RZ), and Manchester are popular serial encoding mechanisms. Each has different characteristics that make it useful in different applications. Those dealing with UARTs will find NRZ to be the most familiar.
There is also a unipolar and bipolar version for many of these encodings. Unipolar has a transition between a zero and positive. The actual measurement can be one of many types of attributes from like voltage, current, pressure, or optical. A bipolar system has a transition between a positive and negative. Any method can employ a bipolar encoding but logically they may be the same as shown with the NRZ example.