Earmarks are a symptom of the problem.

In 2005, the Congressional Research Service found that earmark projects accounted for 1.92% of all federal outlays (spending). Now... that doesn't sound like a lot of money.  Well, OK, in the jaundiced and stratosphere-high world of federal spending, that doesn't sound like a lot of money.  In fact, it would work out to about $47.7 billion dollars, which compared to the $1300 billion that we overspent in 2010, doesn't really seem like much.

However, earmarks are a symptom of a Washington problem that John McCain and a few others have been warning us against for years.  Earmarks are used to buy votes in both houses of Congress.  It's a form of political currency that is used to influence members of both the House and the Senate to vote in a manner that they might otherwise have done.  Here's an example:

House Leader: This reform legislation is very important and we really need your vote in order to guarantee it's passage.
Representative: I realize that this is important to you, but you have to understand that my constituency isn't effected that much by the problem your legislation addresses. And they will object to its cost.
House Leader: Well, what is your constituency interested in?
Representative: We've been trying for years to build that new county library, but the economy has made it difficult to get it done.
House Leader: Well, what if we were to attach, let's say, $600,000 in earmarked funding for that library to the proposed legislation?  
Representative. My constituency will be grateful.  You have my support.

See the problem?  So even though earmarks themselves only represent a small amount of the total federal funds spent, they contribute to a bloated federal budget by making it more palatable to some members of Congress by sending some funding back home where it will do some good.

In the above example, would a new public library be of benefit to the community? Almost assuredly so. But because it's earmarked legislation, it never gets debated as part of the budget.  Therefore, this funding is hard to find and hard to track.

If you get rid of these "pork-barrel" projects and hidden earmarked funding riders, then bills tend to be voted on based only on their relative merit and not because votes have been bought and paid for.  For example, without the sweetheart deals that were made to certain members of Congress in states like Kansas and Louisiana, the United States National Health Care legislation (aka "Obamacare") would probably never have gotten enough votes to pass.

And that's why the Tea Party wants earmarks eliminated.