What is philosophy?
If we start with the Greeks, there's nearly 3000 years of intellectual history. If we start with the figures in the Hebrew Bible, or acknowledge other cultural myths and religions, then this timeline gets stretched at least a thousand years.
The quintessential philosopher is Thales of Miletus, a pre-Socratic touted as the first Western philosopher. He explained nature in terms of ordering principles, instead of solely in terms of mythology and religion. This shift toward natural explanations challenged the status quo of how Greeks understood the world they lived in and their place in it.
Aside from his theory of nature, Thales also shows us some of the mannerisms stereotypical of philosophers. He had a knack for prediction and could excel in practical matters. For example, he bought olive presses before a harvest he had predicted would be successful to capitalize on the situation. However, he preferred the theoretical world of philosophy and once fell into a well because he was gazing at the heavens. So, in the figure of Thales we have someone who was a scientist, a businessman, and an absent-minded sage.
What, then, is philosophy? Great question, and in many respects, the questions are even more important than the answers. Karl Popper once remarked that philosophy was a "second order tradition," not dealing with the first order of definitive answers, but instead passing on a method of critical theorizing. I agree. Philosophy is a way of thinking, a way of trying to understand the world through systematic questioning and through self-attention. It's a discipline that's useless if it becomes esoteric and exclusively self-serving, but philosophy is also crucial for anyone aiming to understand anything within our world.
If this seems interesting, I would suggest doing some further reading. Here's a short list of works by famous authors, sorted by the author's death date. It is biased by my personal taste, so for extensive philosophical timelines I'd recommend: The History of Philosophy without Any Gaps and Kemerling's Philosophy Pages Timeline. Also, if you have interest in a particular topic, try searching google or amazon for "philosophy of x," where x is the topic (e.g. "philosophy of love"). The Blackwell Guides and Oxford Handbook series are great for overviews like this, as are most things Hackett Publishing puts out. A site that may also help is the Stanford Encyclopedia of Philosophy, but it is technical and a bit much for beginners.
Lao-Tzu (531 BCE), Tao-te Ching
Epictetus (135 CE), The Handbook
Marcus Aurelius (180), Meditations
Sextus Empiricus (210), Outlines of Pyrrhonism
Diogenes Laertius (c. 250), Lives of Eminent Philosophers
Boethius (524), Consolation of Philosophy
Anselm of Centerbury (1109), On God's Existence
Maimonides (1204), The Guide for the Perplexed
Thomas Aquinas (1274), Summa Theologica
Niccolò Machiavelli (1527), The Prince
Gottfried W. Leibniz (1646), Monadology
René Descartes (1650), Meditations on First Philosophy
Thomas Hobbes (1679), Leviathan
John Locke (1704), Second Treatise on Civil Government
Jean-Jacques Rousseau (1778), The Social Contract
Voltaire (1778), Candide
Søren Kierkegaard (1855), Fear and Trembling
Karl Marx (1883) , The Communist Manifesto
John Stuart Mill (1873), Utilitarianism
Sigmund Freud (1939), The Interpretation of Dreams
Ludwig Wittgenstein (1951), Tractatus Logico-Philosophicus, Philosophical Investigations
Philosophy in a 2.0 World
The copyright on most great philosophical works has expired, and most philosophers want an audience, so you can find a lot of philosophy on the internet. The below sections are an attempt to try to organize some of the information for you.
Here are some of the most reputable sites for primary and secondary sources of philosophical materials, and they're all free.
Perseus Collection (Greek and Roman source texts and translations)
Philosophy E-server (Canonical philosophical texts)
Project Gutenberg (Extensive collection of literature, including philosophy)
New Advent (Catholic Encyclopedia, with an online version of Aquinas's Summa Theolgica)
PhilPapers (A list of contemporary philosophy and topics)
Philosophy link sites
If you know of anything else, as always, feel free to contact me.
One last recommendation I would give is searching for "Open Courseware Philosophy" online. Or search for "philosophy" in YouTube, but look for schools like MIT, Yale, and Harvard, who have started to post formal lectures on lots of topics for free.