Hi,
is it the same when I say for exemple about 1200: “one thousand two hundred” or “twelve hundred”?
Please, tell me why, and give me some other examples?
Regards.
They are the same; “twelve hundred” means twelve times a hundred. “One thousand two hundred” sounds more formal to me; you would find that written on a bank note.
All the numbers in the thousands are like that. “Five Thousand, Three Hundred and Four” = Fifty Three Hundred and Four". It gets a bit less common to do it the second way past 2000. With even thousands the rule does not apply you would NOT say “thirty hundred” for “3000”.
Street numbers are an exception. If I lived at 1412 Main Street, you would say “fourteen - twelve Main Street” or “one - four - one -two Main Street”. Never “one thousand, four hundred and twelve Main Street” and probably not “fourteen-hundred and twelve Main Street”
In normal speaking, you should get used with “twelve hundred”. I used to get confused in the first time when I was in a conference with some American guys via the phone! Since the maximum bill of USD is 100$ so that it would make it easier to count the number of bills. If you were in Vietnam, it would be in order and our maximum bill is 500.000 VND! 
Some others:
- If $6000: they would say “six K” or “six grand”
That is slang; twelve hundred is regular.