Pressure Bulb
Pressure bulb is a graphical representation of the stresses in a material under a specified kind of loading. The Loading may be a point load, Uniformly Distributed load Uniformly Varying load or any other random loading pattern. It is in form of cuved lines which start from one side of the point of application of load and end on the other side.  So, most of the pressure bulb diagrams look like the drawing of an onion. Peerhaps that is why they are known as pressure bulbs.
The curve nearest to the load represents the maximum stress.
The curve farthest from the poin of application of load is the least stress.
No two curves intersect each other. One curve represents graphically all the points of same stress.
If two or more curves are located near each other, it means that the region is severely stressed.
If the stress lines are placed far away from each other, it means that the region shown is not subjected to higher stresses.
Isobar
Isobar is defined as a line/ curve that represents all the points having a specified stress. It can be of any shape. There can be no cuts or breaks in an Isobar. It can not form a loop over itself. Two isobars of different values never intersect each other. So, you may understand that a Pressure bulb is a collection of many isobars drawn in a sequence.