Used for transporting large quantities of materials over long distances from the oil field to the refinery or from the refinery to consumers.
That which the pipeline might transport could be:
The pipe itself may be configured with all kinds of specifications, such as pipe material & pressure and all kinds of conditions transporting. In some cases the viscosity of the material being transported must be modified in order to pass through the pipe in an efficient manner. For example, Crude oil may need to be heated before passing it through the Trans-Alaska Pipeline to compensate for arctic conditions. To accomplish this they have inline heaters periodically along the route. In the case of heavy oil it may be necessary to dilute it with another liquid to lower the viscosity. It is also possible to move solid rock through such pipeline as in the case of moving a coal slurry where the coal is suspended in water.
Some pipelines were better engineered than others. Some pipelines are better maintained than others. Newer pipelines have sensors placed at intervals to detect changes in pressure and thereby alert of leakage. Older pipelines depend on periodic, sometimes daily, visual inspection. In other words, the entity maintaining the pipe has employees who drive the entire length of it every single day.
There are tens of thousands of miles of such pipelines all over the world. There is a major pipeline running from California to western Texas called the "All American Pipeline".