Externalities are costs in a market not borne by the consumers of the products or services. Pollution is the common example. Waste management (be it unused excess or things like packaging) is another. Anti-competitive practices are a third. In general, they are marked by market distortion.
For example, take waste management rather than some system where corporations have to directly account for product waste. Call a company LeafRing Inc. They manufacture decorative jewelry for you to put on your trees’ leaves. Each piece is sold for $1 and comes in a throw-away wrapper.
Let’s say that suddenly LeafRing had to pay for the waste, all those wrappers, plus damaged LeafRings, plus ones that people quit using. That might raise their unit price to $2, but people will only pay $1. They can’t sell LeafRings, so they go bust.
Instead, we have the externality system, where the true cost is still $2, but LeafRing only cares about the first dollar. The other dollar is up to the whole system (which, it isn’t, due to the way certain government and business funding actually works) to bear.
Basically, the system is designed such that a product that might otherwise be unviable can exist by everyone else paying for certain costs.
Now, that’s not to say society isn’t better off for having LeafRing. Nor to say it is. It’s a tradeoff people can disagree about. And they might choose to call it an effective subsidy instead of an externality.
The problem comes when, in response to externalities, people expect the solution to be free or profit-making.
Take public transportation, for example. The semi-private automobile system is too expensive or otherwise exclusive for some people, and so they need alternative transportation. But enough people get past the velvet rope that public transport is underutilized (a form of de facto segregation, though not entirely along racial lines). The fact that some people are not able to use the dominant transportation is an externality caused by high prices of cars and fuel, plus the demands placed upon operators.
Public transport is effectively the cost of the system we have, but critics often pretend that it should pay for itself. They want a free lunch: cars not having to account for the costs they impose on society, plus the therapy coming at no cost. Indeed, there are car drivers who look at a bus and think they’re paying for transportation twice: once for the car, and once when they pay taxes that support the busline.
In fact, the latter is just paying the hidden costs in the former. And the fact that’s not made clear, that politicians and media portray costs of externalities as though they’re facts of life, rather than as a problem of market distortion, only distorts the political system further.