The word "Singh" is usually pronounced "sing", which is incorrect. The actual origin of Singh is from the Sanskrit word "Simha" or "Sinh" which means "lion". In fact, when writing in the Devnagari script, it is written what would nearest in the Roman script be "Sinh". The British, while in India, coined the spelling "Singh" to represent the sound "Sinh", but users - not knowing the background - recognize the "g" and wonder what the "h" is all about!!!
Singh is used by many people in India and the Indian Sub-Continent, mostly by warrior clans of all sorts. Lately, in a move to make it less apparent, certain persons not belonging to the "original" warlike families have also started to use Singh as their surname, so every Singh cannot claim descent from ancient Singhs.
In certain places, Singh was used as a description for soldiers, with every soldier being referred as a "lion" for motivation of warlike emotions. In most such cases, the actual surname of a person would be another word, with "Singh" being added just before the surname. In time, in some of these persons, the surname got dropped, and they became "Singh". An example here would be us three brothers. My eldest brother writes his name as Inderjit Arora, I write my name as Param Dayal Singh, and our youngest brother: Tilak Raj Singh Arora. Thus, we siblings have "Arora", "Singh" and "Singh Arora" as our third names!!!