The vinegar (a solution of acetic acid) and baking soda (sodium bicarbonate) react to form sodium acetate, water, and carbon dioxide. The carbon dioxide is the gas that fills the balloon.
If you have a bottle and put vinegar or lemon juice in it with water and mix it up good then put a balloon at the top hole fast enough it will blow up because of the carbon dioxide! :) awesomzerzzzzz!