Latex: Standard balloons are made from latex, an organic material, made from the sap of a rubber tree. These balloons are biodegradable and decompose rapidly. Good news for the landfill, bad news for recycling fanatics, as there is no current re-use market for them.
Mylar: Mylar balloons are made with a plastic/nylon, synthetic material that is also recyclable and reusable, but it does not biodegrade. They will be in the landfills forever, so try to keep these for crafts, or recycle them if your program accepts them.