Can someone explain to me what are externalities? (Economics)?

[Kay D]

I understand what they actually are, benefit & cost. But I have not idea on the graphs. I have gone to all websites possible and I have no idea... can you break it down for me. Pleaseee..,.

[Hubris252]

Graphically externalities show up as entirely new curves. The standard supply and demand curves are private benefit and private cost. Positive externalities are depicted as a social benefit curve that is shifted up from the private benefit curve; negative externalities show up as a social cost curve that is shifted up from the private cost curve. The social benefit/cost curves are shifted up from the private benefit/cost curves indicating that the individual either does not enjoy all the benefits of consuming a good or does not bear all the costs of consuming a good. The difference between the private and social benefit/cost at each quantity is the external benefit/cost.

[Hubris252]

Graphically externalities show up as entirely new curves. The standard supply and demand curves are private benefit and private cost. Positive externalities are depicted as a social benefit curve that is shifted up from the private benefit curve; negative externalities show up as a social cost curve that is shifted up from the private cost curve. The social benefit/cost curves are shifted up from the private benefit/cost curves indicating that the individual either does not enjoy all the benefits of consuming a good or does not bear all the costs of consuming a good. The difference between the private and social benefit/cost at each quantity is the external benefit/cost.