"Rent-seeking" is a technical economic term that doesn't mean what most laymen might expect, but that when understood can illuminate a large portion of our political and economic system.
The political left is generally more fond of empowering the government than the right; most people are willing to admit this, no matter what their own political persuasion, but they see the effects of it differently. Someone on the left will think a powerful government is important because it allows the public to care for the needs of the poor and to provide for the public good. A leftist generally won't understand why anyone on the right would think this to be a bad or scary prospect, but an explanation of "rent-seeking" may reveal the bogeyman those on the right are afraid of.
Simply put, rent-seeking -- in the negative sense -- is the use of political power to force others to pay you rent for the use of something that doesn't belong to you -- imagine me setting up a toll booth on the Golden Gate Bridge. This would naturally be illegal, but the problem with large and powerful governments is that lobbyists push for (and get) laws enabling them to do exactly this sort of thing.
For instance, when the government pays subsidies to domestic sugar producers and imposes high tariffs on sugar imports, sugar consumers end up getting ripped off -- we pay not just for the subsidies (through taxes), but we pay more for the sugar itself because of reduced competition. The government essentially allows the sugar industry to set up a toll booth on our borders, and charges us for the privilege. (And the rent-seeking hurts poor foreign sugar producers even more.)
Kelley L. Ross wrote an excellent essay called "Rent-Seeking, Public Choice, and The Prisoner's Dilemma" which opens with the quote: "If you rob Peter to pay Paul, you've already got half the vote. -- Aegyptophilus." That's why rightists fear an unlimited government that isn't restricted in what it can do. "With money we will get men, said Caesar, and with men we will get money." Money is used to buy votes, and those votes are used to transfer more money to the people who vote the "right way".
No matter who runs the government, Republicans or Democrats, the system will be abused. It's immoral, but it's inevitable. Power corrupts. The way to reduce the corruption in government is to reduce the power of government. As is becoming increasingly clear to those who didn't believe it at first, so-called Campaign Finance Reform is entirely useless. It isn't reducing the amount of money being spent, and it isn't even limiting how and when ads are used. Michael Moore's stupid new movie is one giant attack ad against President Bush, and it's completely outside the scope of CFR. Do we need yet another law to cover movies? Then what, a law against newspapers? Blogs? It's absurd.
Reduce government power, and you reduce the incentive for people to lie, cheat, and steal to get their hands on that power.