A public good is a product or service that is made available to all members of a society. These goods are characterized by two main features: they are non-excludable, meaning that no one can be effectively excluded from using them, and they are non-rivalrous, meaning that one person's use does not reduce availability for others. Examples include clean air, national defense, and public parks. Public goods are often provided by the government or through collective efforts because private markets may not supply them adequately. This is due to the difficulty in charging individuals for their use, leading to what is known as the free rider problem.