Nowadays it seems like everything has a "philosophy". Marketing campaigns, people, companies - they all have "philosophies". Or rather, they think they do. Philosophy literally means "love of wisdom", but it is a term that has become overloaded with all manner of extra connotations and meanings over thousands of years. It means a bit more than simply holding opinions that you can defend. What it originally denoted when philosophy began is a relatively simple idea, if perhaps now a counterintuitive one.
Philosophy is the human capacity which aims to investigate questions that are of concern to man as man. This means that the issues it investigates are both timeless and transcend cultures. A concern with ideas that are specific to a particular culture and time are instead the hallmarks of what we call the "intellectual". The difference between Plato and an Ancient Greek intellectual is that Plato addressed topics that will concern man for evermore, which is why he is still read avidly today. And while these questions concern all cultures, their investigation via philosophy is not something that has occurred in all cultures - it is rather the unique contribution of the Greeks to western civilization.
Throughout history most cultures have addressed questions of first principles by reference to either gods or ancient heroes to whom they attribute their beginnings. Earlier generations were presumed to have had privileged access to divine knowledge which is now embodied in either scripture or tradition. Philosophy emerges when people notice the multiplicity of traditions and religions in the world and take it upon themselves to work from this diversity of opinion about questions such as how life ought to be lived and to try and work out, through their own capacity to reason, what the answers to various questions about life and the universe are.
Each of the branches of philosophy has at its core a question or set of questions that are of concern to man as man. For instance, political philosophy asks: What is freedom? What is justice? In a way, this is the most practical of all branches of philosophy because these are questions that every society must answer explicitly; and, they are questions that will concern people for as long as they must live together. They are hence the proper sphere of philosophy - an investigation of the first principles of man as a social and political being, and of the various possible ways in which people can live together. The limits of political philosophy are coeval with all the possible ways that politics might be organized, and it conists of a consideration of these possibilities.
To take another example, metaphysics asks: What is the nature of Being? "Being" sounds rather outdated now, and a more modern way of stating it is: What are energy and matter? For the longest time men sought the solution to this problem through the power of their minds alone, constructing grand theories from the ether - the idea of the atom originated in this fashion in Ancient Greece. But metaphysics has fallen severely out of fashion of late because it has been overtaken by empirical science, which in itself involves the philosophical choice of choosing to investigate the world through our sense-perception. Indeed, science can be viewed as the most successful branch of modern philosophy, as its efforts to ascertain the true nature of Being through empirical investigation rather than abstract metaphysical theorizing have delivered real results. And clearly, the true nature of energy and matter are of concern to man as man regardless of his culture and the period in which he lives.
Philosophy is then the human capacity which allows us to use our reason to transcend our immediate surroundings and turn ourselves to these larger questions. As such, it involves an alienation from one's immediate surroundings and the ability to question received wisdom. This is perhaps where philosophy derives its reputation for being irrelevant or somehow merely academic from - it certainly does not appear, at first glance, to address any concrete practical concern faced by people in their everyday lives.
Yet there is arguably no such thing as a free life that does not concern itself with the first principles of things and instead accepts them unthinkingly from its surroundings. And so the call has gone out from philosophers down the ages - go out and investigate the realm of ideas, but don't expect to be too welcome in the cave of real life if you want to come back burdened with what you have discovered.
The modern turn against philosophy is in a large attributable to the success of a specific form of knowledge, science, and its technological ramifications. A specific set of philosophical choices about the nature of the universe and of human knowledge have become so dominant and delivered such amazing results in terms of transforming the way that our societies function that all other possibilities seem to have sunk into the background. But even for Nietzsche to be able to declare that God is dead - that from now on we repudiate any belief in the immaterial, in philosophy's grand theories as much as in God - was itself a philosophical enterprise, an altering of ideas about the first principles of metaphysics, human living-together and knowledge.
These new assumptions serve us fine for now and appear to have vanquished in most people's mind the need for further philosophical speculation, except at the margins. We resemble, and not a little, the pre-philosophic peoples who place all their faith in the teachings of the past. But in a new world - a world, perhaps, of peak oil and climate change, or one dominated by a China which has developed without the influence of our western philosophy and has ideas very different of its own - we will need new philosophies and new investigations, for the engine in the current motor of our societies will have run out of gas. It is the comfort of philosophy students that as this time approaches, they are the gatekeepers of mankind's investigations into the first principles so far - just as the Arabs were during our Dark Ages, before transmitting Europe's past to her progency - and thus the hope for its continuation in the future.