Earmarks are defined as "Provisions associated with legislation that specify certain congressional spending priorities or in revenue bills that apply to a very limited number of individuals or entities."
Pork Barrel usually refers to "Spending that is intended to benefit constituents of a politician in return for their political support, either in the form of campaign contributions or votes." It supposedly originated in a pre-Civil War practice of giving slaves a barrel of salt pork as a reward and requiring them to compete among themselves to get their share of the handout.
Both terms are derogatory and used interchangeably. Either are requested by only one chamber of Congress; not specifically authorized; not competitively awarded; and serve only a local or special interest. Below are a few examples.
$1,800,000 was earmarked for a climate model evaluation program. This is on top of the $2 Billion in stimulus money for the climate science program, and another half a billion in stimulus money that the White House directed to global warming. I am getting warm just thinking about it.