topping up the other answers by the following :
since the public good is the type of good that every one has a right to be benefited from and have reasonable access to it , they are not favorite type of good for the private sector to invest on . that is why one the most important jobs of governments is to provide public good to the nation since there are no private sector interest on it due to the fact that the production of public good does not create high rate of return for the investor.

A public good is a good whose use by others is not diminished by any person. Specifically a good is a public good if no one could be excluded from use by some action of another and the good is not rivalrous, in other words the use of one does not diminish the use by another.

An example of a public good is national defense. Your receipt of national defense is rival with others and isn’t diminished by your neighbors use. Another example is the light from a street lamp.

Similar to this is a “common good,” such as fish in the ocean. Catching a fish means the fish is diminished by others, but you cannot keep other fishermen out of the entire ocean.

Another similar one is club goods. Cable television is an example of a club good. No ones usage is diminished by other people using cable, but you can exclude people from cable television such as if the cost exceeds the budget constraint of a family.

A private good is a good whose usage diminishes the usage by others and where your usage could cause the exclusion of others or which you could simply exclude others. A simple example is a restaurant with one pie with eight pieces. If a party comes by and buys up the eight pieces of pie, should a ninth person come in to order it, rivalry has already consumed it. Likewise, if the pie is sold at a price which prohibits some people, or if it were sold in a woman’s club then exclusion exists.

Public Goods: Water
Private Goods: iPods