A Proper Noun is a special type of noun which refers to a person or place (and sometimes a title).
Tom Cruise, Spain, Alexander the Great, Pluto...
Proper nouns help identify a particular person, place, object or animal because they tell us their name.
Compare these statements:
I met a girl yesterday
(very general information, which girl do we mean, it could be one of many)
I met the girl next door yesterday.
(more descriptive information, I've narrowed it down a bit)
I met Sue yesterday.
(precise information, we both know who Sue is so we know exactly the girl we're talking about)
As with people, when we talk about a place which does not have a name, we use a common noun; if it has a name we use a proper noun.
For example, we say:
prison, hospital, house
but we talk about
Wandsworth Prison, St Guy's Hospital, The White House
Proper nouns always begin with a capital letter, no matter where they are in a sentence.
Queen Victoria, Cleopatra, Buddha, London, Australia, Mars, Asia, Freddie Kruger
Cleopatra was the last active pharaoh of Ancient Egypt.
The Roman emperors Julius Caesar and Mark Anthony both fell in love with Cleopatra.
It is said that Cleopatra died from an asp's bite.