Electrochemical cells have at least two electrodes, and those cells used for analysis purposes usually have three electrodes. While various redox reactions occur at each of these electrodes, there is usually one particular electrode in the cell which is the focus of the experiment or analysis in question. This principle electrode is called the working electrode.
In a typical three electrode voltammetry experiment, there is a working electrode, a reference electrode, and a counter electrode. A redox process of interest is studied at the working electrode by controlling the potential of the working electrode with respect to the reference electrode while measuring the current at the working electrode.
Some electroanalytical cells have more than just one working electrode. A very common example is in rotating ring-disk voltammetry, where both the ring electrode and the disk electrode are considered to be working electrodes.