気球 http://www.google.com/search?gcx=c&q=%E6%B0%97%E7%90%83&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&biw=1280&bih=702 風船 http://www.google.com/search?gcx=c&q=%E9%A2%A8%E8%88%B9&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&biw=1280&bih=702

気球＝hot air balloon 風船＝balloon

In Chinese, 気球 is a balloon and 風船 is an airship, powered by hydrogen.

気球　is usually used for observation of something or carring people. 風船　is usually usedo for just a toy or decorations. Above sentence is just a example, so that there are eceptional case, for exmaple, big or a lot of 風船　can carry people. Since it is difficult when we talk about strict definition of them beause both's definitions are almost same that example or image will be help. image 気球　is flying in the sky for purpose of something and structure of it is more complex than 風船. 風船　is rubbar toy or decoration which contains light gas in it.