Example : I am living at Vietnam. I want to advertise Adwords at Austrailia (google.com.au) but dont want people in Vietnam see my ads if they search on google.com.au . How I do it ? Thank you very much.
When you set up your Adwords campaign you get to pick where your ads show. So you really should not have to worry about that issue.
Exactly, just geotarget wherever you want your ads to show. You can do by country, city, radius, hell you can even draw out an area in the adwords interface on a map.