A field of study which can essentially be divided into the subfields of epistemology, metaphysics and ethics.
Epistemology is the study of knowledge, how it is gained and to what degree it is possible.
Metaphysics is the study of what exists, how it exists and how we define it.
Ethics is the attempt, having decided upon theories of knowledge and existence, to create a coherent way of living.
Contrary to popular belief, philosophy is not, or at least does not have to be, useless. If everyone thought carefully about what they believe, why they believe it and how they should act accordingly the world would be a better place. Everyone from politicians to scientists to soccer-moms must make decisions of ethics which affect them and those around them.
Sartre (a major figure in philosophy) says of epistemology that we cannot have absolute knowledge, of metaphysics that the world is subjective, and of ethics that by existing we choose ourselvse and the world.