In what follows, as in my blog generally, the term "philosophy" refers to Western philosophy. This is not because I have anything against philosophy from China or India or the indigenous cultures of the Western Hemisphere or from anywhere else on the planet; on the contrary, it's merely because I know so little about non-Western philosophy.
Every now and then someone who knows a bit about physics or biology or geology and remarkably little about a lot of other things will answer the question "What is philosophy?" by saying that philosophy was what very weakly and incompletely plugged a few gaps in things before Francis Bacon formulated the scientific method and the Scientific Revolution got underway, and add with a condescending smirk that of course this answer doesn't sit well with philosophers. And of course he or she (usually he) is right, that answer does not sit well with philosophers. Or with anybody who actually knows what philosophy is, or knows that the scientific method actually was used now and then -- by philosophers -- for thousands of years before Francis Bacon formulated it.
Western philosophy is the pre-Socratics, Socrates, Plato, Aristotle and their pals, and Russell, Wittgenstein, Sartre, Rorty, Sloterdijk and their pals, and all the people in between who studied Plato and Aristotle and the people in between up until their own time, however you want to describe them, and whatever their attitude toward their illustrious predecessors was.
Within Western philosophy, up until and including Galileo and Francis Bacon, the terms "philosopher" and "scientist" mean pretty much the same thing, and since then, by and large, with a few exceptions, philosophers have tended to know a shitload more about science than scientists have known about philosophy. People generally these days have a healthy appreciation of and respect for science, and philosophers are very rarely an exception to this rule. It's a shame that some prominent scientists don't know jack about philosophy, or history, or art or literature or music or psychology, and yet publicly hold forth on their special area of ignorance as if they had a clue. That's really a shame.
I am notorious for my unwillingness to describe philosophy any more exactly than by saying that it's been what has been written by those people known as philosophers: the pre-Socratics, Socrates, Plato, Aristotle, Epicurus, Zeno, Boethius, Roger Bacon, William of Occam, Machiavelli, Copernicus, Francis Bacon, Pascal, Hobbes, Descartes, Spinoza, Leibniz, Locke, Berkeley, Hume, Kant, Hegel, Fichte, Marx, Schopenhauer, Nietzsche, Wittgenstein, Heidegger, Russell, Sarte -- you know, all those guys plus the handful of gals which they, unfortunately very misogynistic, very many of them, have let into their club. What it is is what those people have done, however you choose to describe it. I don't define it any more exactly than that because I happen to think that that is exactly what it is. If you have something intelligent to say about Plato and Hume, because you've actually read and understood them, there's a good chance you could reasonably be called a philosopher. If you can't, because you haven't, then chances are you can't be. Just as I think that a religion is that group of people who identify as belonging to that religion, and not a set of beliefs, so I think that philosophy is the above-described group of people, and not something they all have in common which can be abstractly described in several dozen words or less. Philosophers, including the famous ones, have been brilliant and stupid, gregarious and misanthropic, nationalistic and enemies of cultural boundaries, polyglots and bigoted haters of all but one language (although more usually polyglots), world travelers and agoraphobes, sometimes not misogynistic at all, sometimes women, and so on and so forth. As we all know, "philosopher" means "lover of wisdom," but it's not as if there's anything close to a consensus among philosophers about what actually is and isn't wise, or who's wise and who's a fool.
I suppose I actually can think of one characteristic which philosophers generally share, just one: we like to read the works of other philosophers, even the ones we disagree with intensely. We read the latter so as more soundly to refute them and overturn the influence of their folly. It's not as if we're doing it for the money.