ma’am

The only place where ma’am is used to address any woman is in the US. Outside of the US it's chiefly used to address royalty.

http://www.oxforddictionaries.com/definition/english/ma%27am

Also, there is a difference in pronunciation between UK and US see: http://english.stackexchange.com/questions/12113/what-is-the-correct-pronunciation-of-the-word-ma-am

madam

The term madam can be used anywhere ‘to address or refer to a woman in a polite or respectful way’.

http://www.oxforddictionaries.com/definition/english/madam?q=Madam#madam__4

ma’am

The only place where ma’am is used to address any woman is in the US. Outside of the US it's chiefly used to address royalty.

http://www.oxforddictionaries.com/definition/english/ma%27am

Also, there is a difference in pronunciation between UK and US see: https://english.stackexchange.com/questions/12113/what-is-the-correct-pronunciation-of-the-word-ma-am

madam

The term madam can be used anywhere ‘to address or refer to a woman in a polite or respectful way’.

http://www.oxforddictionaries.com/definition/english/madam?q=Madam#madam__4