The pH electrode depicted above is a single junction glass bulb electrode which is the most commonly used pH electrode in industry. There are many other variations including double junction, antimony and specifically formulated glass for various environments.
Double junction electrodes are the most common variation from the probe depicted above. This electrode simply provides a second chemical barrier between the reference electrode and the process fluid. This is done to help slow down the effects of electrode poisoning.
pH Probes are inherently unstable and require frequent cleaning and calibrating. A discussion of pH probe cleaning and calibration is available within this site. Although this procedure is rather generic it applies in principle to all pH probes.
Also available is a less technical and more practical writeup covering pH probes, transmitters, and pH controllers.