Hi! I'd like to know which is the usage difference between

  1. a hundred / a thousand / a million

  2. one hundred / one thousand/ one million
I'm a little confused because a vocabulary dictionary uses "a" instead of "one" while other sources use "one".

Thanks a lot for helping.
The way I see it...
When you say 'a hundred' or 'a million' you're just giving an order of magnitude, and the exact figure doesn't matter. So much so that when you say 'a million reasons not to do this' it obviously isn't important how many exactly.
When you say 'one million' or 'one thousand' then you're usually being quite precise with the figure you're giving : 'I've won a million pesos at the lottery' (lucky you even though I suspect this isn't that much) or 'This village is one thousand inhabitants'.
Sanzdifference between
1. a hundred / a thousand / a million
2. one hundred / one thousand/ one million
No difference. A hundred is one hundred. One hundred is a hundred.

Use "one" instead of "a" when working with numbers as an accountant, mathematician, or scientist might do.

Teachers: We supply a list of EFL job vacancies
One hundred and a hundred are the same number. If you look at the definitions of "a" one definition is "one". "a" indicates a single unit. If you take hundred out and try other nouns you might see that it is this way. When speaking you will find that most people use a instead of one. The rythm seems better and it is easier to pick out the important parts. For example: I need a dozen eggs, a loaf of bread and a gallon of milk. Very few would say one for these items, it just sounds wrong. With the push to leave out extra letters (like only using a single space after a period) one would think "a" would be preferable to "one". I do not find anything specific in MLA, APA or Chicago style that says what is right for which circumstance.

If I were to say I have a million dollars or that I have one million dollars I have not changed anything. This is a lot like wheather it is pronounced a or uh, do what seems right at the time.
 CalifJim's reply was promoted to an answer.
Site Hint: Check out our list of pronunciation videos.
Dear CJ,

I believe that colloquially, interchanging 'a hundred' with 'one hundred' would be ok if you were an accountant, mathematician, or scientist because it does not change the number, but if you are a writer perhaps, or other type of proffessional that deals with grammar and the English language, then you would want to be correct in writing or speaking rather than mathematically. I have been trying to figure out what the difference is between 'a hundred' and 'one hundred' for a good part of this day only because I want to prove wrong what it says on the vanilla visa website: 'one card. a million options.' which is actually terrible anyway because neither of those are complete sentences. The only thing I have come up with is that 'a million' is more general and 'one million' is more specific. For example: "There are a million trees in my backyard!" versus "One million pennies remain in your bank account."

Anyway thanks to all who reply as I would love to find an answer to this question.