The terminology among Cannabis users does get confusing until you get your head around it. Basically, a glass bong is a device for smoking dried flower and an Oil Rig (also called a dab rig) is for smoking concentrates. Most glass bongs can be converted to oil rigs with the addition of a quartz banger. Glass Bongs and oil rigs are normally manufactured in Pyrex glass or high-quality food-grade silicone.
Glass bongs come in all sorts of shapes and sizes see types of percolator bong (link). Bongs, as stated above, are used to smoke flower. The weed is placed in the cup, lighted with a match, lighter or butane torched and as you inhale is drawn through the water and into your mouth. The water adds cooling and filtration to give a cool clean hit.
Oil Rigs are similar in that they are used to draw the smoke through the apparatus, but they differ in that a small amount of concentrate is placed in the banger, and heated to the point of vaporization before being drawn through the piece.
In general terms, glass bongs are usually larger, except of course when one is converted to a rig. They can be anything from mini bongs, which will fit into your pocket, up to 50 cm tall scientific masterpieces that are both elaborate and superbly efficient. Oil Rigs are normally smaller more compact pieces of kit.
An oil rig setup will normally cost a bit more than a simple bong. This is mainly due to two factors, First, often you are buying a glass bong and then adding the rig by way of a quartz banger, Second in addition to the rig itself you will need to buy other dabbing tools, like a dabber, and a container, etc.