In economics we may identify different types of goods, for example private goods, and public goods. There are others but in this video we will focus in on public goods. These are goods that have special characteristics, for example consumers cannot be stopped from consuming them , and also the more that consumers use the goods does not diminish the amount left. An example of this could be St lighting.
Externalities our interdependencies associated with goods and services for example if you have a party and play a loud music you may enjoy that, however your neighbours may not do so. In this case your neighbours have what we call a negative externality. They have a cost and you do not compensate them for that cost.