Expecting an answer to the question "what is a liberal" on this board would be like asking me what a conservative is. I could only answer out of my own prejudices, as I've never been one.
Here's what I, and many of the other liberals I communicate with, believe in:
1. Regulated capitalism. Unrestricted anything tends to be bad, as it magnifies the "dark side" along with the good. Without effective regulation, capitalism degenerates into the kind of ruthlessness we saw in the Industrial Revolution. I, for one, have no desire to live in the 1880s. Left to it's own devices, capitalism tends towards oligarchy and oppression. This was the one thing Marx got right. (His solutions, on the other hand, are simply nonsense.) A regulated marketplace, which serves to limit the power of any one participant, is, we think, the best way to ensure the benefits of capitalism while avoiding it's worst potential abuses.
2. Labor unions. It is only through collective action, and negotiating as a group, that workers can begin to equalize the power relationship between themselves and their employers. It is in the employer's interest to extract the maximum work for the minimum cost. It is in the employee's interest to get the maximum benefit for the work that they do. In the inevitable conflict, liberals tend to take the side of the employee.
3. Separation of Chuch and State. I want my government to be completely silent when it comes to religion. Religion is a private matter, one which the state should stay 100% out of. Theocracies are inevitably oppressive. Religion is a force of great power. So is the state. Keep them apart.
4. Universal health care. For most liberals, this is a moral issue. Poverty shouldn't be a death sentence. Human life and health are not "products", and I don't want the market deciding who lives and who dies. It's possible to argue this one on the basis of cost, as well, but for most, it's ethical, not financial.
5. Limitations on the power of money. Especially unearned money. This is where the idea of inheritance taxes comes in. Being born rich confers enormous advantages, which the individual so blessed has done nothing to gain for him/her self. This is also where ideas like campaign finance reform come from. From the liberal perspective, one's net worth shouldn't confer any greater political power than any other citizen posseses. Ideally, it shouldn't give you any advantage other than the ability to buy more stuff, but that's kinda unrealistic.
6. Equality. Essentially, the idea that, from the state's perspective, all citizens are the same. This is not "Robin Hood." It is, again, a desire to minimize power differentials whenever possible.
7. Privacy. The decisions I make, so long as they impinge on no one else's liberties, are no business of the state. What I put in my body, who I have sex with, who I live with, what I do to myself is nobody's business but my own.
8. A general distrust of absolutes. There are exceptions to this, of course, but most liberals I know get pertty suspicious of anyone claiming to have "the one true way."
9. Distrust of power, and a reluctance to use it, or see it used in our name.
10. No short description here, but most liberals I know don't subscribe to the "Shining City on the Hill" idea. Which isn't to say that we're anti-American. For the most part, we'd just rather that America minded it's own darned business. There's noting special about us, as a people. We have inherited a (mostly) great society, but there's no reason for that other than the accidents of history, and we have no right or obligation to make anyone else be like us. If they want to, fine, but I really don't care.
11. Dissent. Disagreeing with one's government is not only the right of every American, it's a duty. Those in power should not be trusted, simply because they have power. They work for us, and we need to monitor their use of the power we give them closely, or they will misuse it. The freedom to disagree is the bedrock of liberty. Without it, nothing else matters. That's why they put it first in the Bill of Rights.
And finally, most of us are patrioits. You can, of course, believe that liberal=traitor all you want, but you'd better not even think it real loud around me. Veterans tend to take that rather poorly.
You'll notice that there's nothing in here about gun control. That's because most liberals I know don't believe in it much. Now, certain politicians do, but I think that has much more to do with being "tough on crime", whatever that means, than liberalism, per se. For what it's worth, gun control hasn't been on the Democratic Party platform for a long time now, and the Party's chairman got a 100% rating from the NRA when he was a Governor.
Hope that helped.
--Shannon