'Najaf' or 'An Najaf' ?

[djbdjb]

I'm following along using National Geographic Atlas 7th ed.
It lists the names of cities as follows:


As Nasiriyah
Al Basrah (has Basra in parentheses)
As Samawah
An Najaf
Al Hillah
As Sulaymaniyah
Az Zubayr

I have seen some of these cities named without the preceeding word. Are both correct, or are some U.S. news orgs. a little lazy?

What are the Arabic meanings of Ad, Al, An, As, Ar and Az ?