“Earmarks” are items on congressional spending bills where the funding goes to specific, localised projects in either one state or a few states (more bridges for the Ohio river) or for a very specific group of people (one-legged teenage moms in Nebraska). The “Earmark” is typically either a “pet project” of a specific lawmaker or grease for a lawmaker’s local community (electorate) in a state or part of a state. An earmark is also called “pork” (the Danish term, used somewhat differently, is “valgflæsk”).
Economically, earmarks are spending that would not take place if the beneficiaries of the earmark had to spend the money themselves. An earmark can only become a reality because the cost of the earmark is spread out on all taxpayers. Of course, this goes for most government spending, but earmarks are the extreme text-book case.
Politically, earmarks become a reality because Congress doesn’t vote on spending one item at a time, but rather vote on gigantic bills full of hundreds or thousands of items. In this way earmarks find their way into the bill as a way of buying favor among the Congressmen. “You support my California windmills and I will support your bridge-to-nowhere in Alaska”. It is a common feature of political life in all democratic states, not only in the US.
Additionally, the advantage of including earmarks in one big spending bill is that if the President wants the earmark out of the bill, he will have to veto the whole bill or send it back to Congress. This is usually too much trouble, so the earmarks stay even if the President doesn’t want them there. After all, there’s so many other good things in the bill.
Earmarks are a byproduct of the way democracy works. See Public choice theory.