No, they are not the same thing, although sometimes a one can cause the other.
A murmur is caused by turbulence in the flow of blood in the blood vessels and parts of the heart. Wherever there is a narrowing, a kink in a vessel, an opening which conducts blood in an abnormal way, etc., there is the potential for a murmur. Think of a garden hose: if you run water through it, usually it does not make any (or at least not very much) noise. However, if you fold the hose like you're going to stop the flow, it starts to make a "shhhh" kind of noise at the kink. (Actually, the "shhhh" noise you make with your mouth to silence someone is caused by turbulent flow over your teeth and lips, but that's another article).
In the heart, there are two upper chambers called atria which help load blood into the lower chambers (the ventricles). Between each atrium and its corresponding ventricle is a one-way valve (the mitral valve on the left, tricuspid on the right). On the way out of each ventricle to the arteries is another valve (the aortic valve
on the left, the pulmonic valve on the right). If a valve is too tight (the leaves of tissues are stuck together partially, you have something growing on the edge, or have had certain diseases which damage the valves), the blood going by the valve makes noise going in the forward direction. If the valve leaks blood backwards when the heart contracts or relaxes, the backward flow of blood can make noise in the opposite direction.
Also, sometimes there is a hole in the wall of the heart (the septum) between the left and right atrium
(an atrial septal defect
, or ASD) or between the ventricles (a ventricular septal defect
, or VSD). Because the pressure on the left side of the heart is usually so much higher than the pressure on the right, this can cause abnormal flow across the defect, which can also cause a murmur.
Finally, there are certain murmurs
which are usually only heard in newborns, in most cases disappearing within a few hours to days after birth. Because the lungs are not used in fetal life, the pressure from the right side of the heart is actually as high or higher than from the left at that time. There is a connection between the pulmonary artery
(the exit from the right side of the heart to the lungs) and the aorta (the artery to the rest of the body from the left side of the heart) that allows most of the blood to go around the lungs. This connection is called the "ductus arteriosis" (Latin for "duct of arteries"), and it is supposed to close with a day or so after birth. As it is closing, there is more turbulence at the connection, so it can make quite a loud murmur.
In some babies, the duct does not close all the way, and the newborn continues to have a murmur. Because the pressure is higher on the left than the right, this pressure affects the lungs and the right side of the heart, and the right side of the heart can enlarge, which is bad in the long term. This is what I meant about murmurs causing enlarged hearts (actually, the pathology causing the murmur also causes the enlargement). Similarly, an ASD or VSD can cause pathological enlargement on the side which is supposed to be lower pressure, the right after birth and the left before the baby is born. Also, if the aortic valve, for example, becomes sticky later in life due to illness, the increased pressure in the ventricle before the valve can cause the ventricle to enlarge, just like high blood pressure
except the pressure is just inside the heart and not necessarily in the blood vessels.
Other processes can enlarge the heart, typically high blood pressure in older adults. This can enlarge the valves or otherwise damage them, leading to a murmur caused by enlarged heart.