The first section of hazard is the early failure hazard, where the hazard rate is very high initially but goes on decreasing steeply with time as the defective products are identified.
The second section has almost a constant level of hazard as it is mainly due to random errors present in the products.
The third and the final section is referred to as the wear-out error where the hazard goes on increasing with time. It is mainly the period after the design lifetime of the product and thus the increase in hazard.
An image of a typical bathtub curve is as below and it derives its name from the cross-sectional view as is visible below.