Paul Krugman, in a very perceptive post, not only gives a very good definition of what Social liberalism is, but also a most eloquent rationale for why it is the most decent political "ism" out there:
[T]he typical conservative line about equality of opportunity, not results, really implies the need for a radical restructuring of our society, which doesn’t offer anything remotely resembling equal opportunity. At this point, however, there’s a tendency to think about what that restructuring would involve — and because it’s basically impossible, to throw up one’s hands.
The point is that you don’t, in fact, have to be that radical once you drop the rigidity of the conservative position. If you admit that life is unfair, and that there’s only so much you can do about that at the starting line, then you can try to ameliorate the consequences of that unfairness.
My vision of economic morality is more or less Rawlsian: we should try to create the society each of us would want if we didn’t know in advance who we’d be. And I believe that this vision leads, in practice, to something like the kind of society Western democracies have constructed since World War II — societies in which the hard-working, talented and/or lucky can get rich, but in which some of their wealth is taxed away to pay for a social safety net, because you could have been one of those who strikes out.
Such a society doesn’t correspond to any kind of abstract ideal, whether it’s “people should be allowed to keep what they earn” or “from each according to his ability, to each according to his needs”. It’s a very non-Utopian compromise. But it works, and it’s a pretty decent arrangement (more decent in some countries than others.)
That decency is what’s under attack by claims that it’s immoral to deprive society’s winners of any portion of their winnings.
© 2011 The New York Times Company.
And since it's just a couple of days since the horrible tragedy in Arizona, I might add that while mentally disturbed murderers of politicians often have famous and/or infamous political tracts at home, and quite possibly as favorites, there are certain political tracts that don't seem to be as popular with the psychos, terrorists and killers out there, whatever their political leanings. Social democracy and social liberalism, for example, don't seem to move very many people to severe violence. John Stuart Mill's "On Liberty", which can be seen as the first great work of social liberalism, is one of those books that doesn't seem to elicit violence. Not just for that reason, I warmly recommend it – it's a very persuasive, and largely very modern, book.
It's not perfect, however (and I hope to come back to the subject of how the search for perfections actually seems to be an indication of a totalitarian mind-set) – because however intelligent and far-sighted Mill was, he was still a man of his time, and he didn't see colonialism in quite the manner we see it today. So he was flawed, but let's all just grant that he was wrong on that issue and not make up some long-winded defense of him there: he was simply wrong. He was, however, as close to flat-out correct as it gets when it comes to the subject of liberty, and the book is warmly recommended.
(I may take myself up on that recommendation, as it's probably been about a decade since I last read it.)