MSG, or monosodium glutamate, is a controversial salt of sodium and the naturally-occurring amino acid glutamate, refined from naturally occurring glutamate. The natural form of glutamate is found in many foods, and when 'free' (not bound to a protein) it enhances savoury flavours, which is called umami in Japanese. MSG is a chemically refined form of glutamate, bound to salt.
Since MSG is a sodium-containing salt, many of the health concerns associated with table salt also apply to MSG, but many people have reported additional negative experiences. Some of these apparently-MSG-specific problems are experienced as a short-term 'reaction', whereas most concerns about salt are over a longer time frame. Many authorities disclaim health concerns over MSG, and on the other hand, many consumers of the substance claim first-hand experiences which resound widely. It may therefore be inferred that a certain amount of disinformation or misinformation exists regarding this widely-used product. Care should be taken in evaluating the issues surrounding MSG, and sensitivity provided to those who believe their real negative experiences are caused by the substance, whatever may be the technicalities.
Many people with migraine headaches report MSG as a trigger substance.
The use of MSG can also actually reduce total sodium content in food, because the food may not need as much table salt to taste good. A small amount of MSG might typically reduce salt usage by 20% to 45% without loss of flavour, and this might be seen as a health benefit, if no negative effects occur due to MSG.
Glutamate in some form is found naturally in seaweed, in fermented soy products such as miso and soy sauce (1.0%), and especially in yeast extracts like Vegemite (1.4%) and Marmite (2.0%). Small quantities are also present in tomatoes, mushrooms, and Parmesan cheese (1.2%).
Glutamate being produced for refinement into MSG is most-frequently commercially produced via fermentation; the result of this stage of the production process is 100% identical to the glutamate found naturally in many foods. Another path is by hydrolisis of gluten, eg from wheat.
"The crude glutamic acid produced in this process is then filtered, purified and converted by neutralisation into monosodium glutamate. After additional purification, crystallisation, drying and sieving, monosodium glutamate has the form of pure white crystals ready for packing and use." 
Food products sometimes show ingredients like "autolysed yeast" and "hydrolysed protein", without necessarily using the word MSG or the recognised numeric codes (E621, HS29224220) on the label. These ingredients may in fact contain MSG, or at least free glutamate. There is some confusion as to whether health concerns correlate with free glutamate as well, or just with MSG. It is unlikely that protein-bound glutamate is problematic for the vast majority of people, as this would be absolutely crippling.
MSG often is used by adds extra flavour to snack foods, frozen dinners, and instant meals, for example the seasoning mixtures for instant noodles.