Like a steel cylindrical pipe, an artery is comprised of an inner space (the "lumen", filled with blood) enclosed by a wall ( you can go to the section on Brain Artery Structure by clicking here). The wall is made up of a number of layers, two of which are muscle tissue and elastic tissue. When a region of the blood vessel wall weakens, it can balloon out to form a sac-like structure. This structure is called an aneurysm (a word derived from the Greek, aneurysma - a widening), and the major problem associated with aneurysms is that they can rupture, an event which may be fatal.
At the outset, it should be noted that there are different types of brain aneurysms. They are usually classified as being either "true" or "false" aneurysms.
A true brain aneurysm is an expansion of a blood vessel wall involving all layers of the wall. The two most recognized types of true aneurysms are "saccular" and "fusiform", although a third much rarer type called "mycotic" is also recognized: