Metaphor is usually seen as an aspect of words, a linguistic trick we use to increase the effect of our words. Lakoff sets out to show that metaphors are a fundamental part of our thought processes whenever we try to think abstractly. His book does not provide a rigorous scientific proof, but it does present a lot of evidence in favor of the thesis. However, a full treatment of the issue would take a much thicker and less readable book than this one.
Lakoff gives examples from life for various metaphors, for example, TIME IS MONEY (or TIME IS A VALUABLE COMMODITY), and shows how we use these metaphors in our everyday thoughts and actions ("Spending time", "wasting time", "saving time", etc). He shows how many different ideas can be expressed with simlar metaphors, ie HAPPINESS IS UP / SADNESS IS DOWN, HEALTH IS UP / SICKNESS IS DOWN, and so on.
Lakoff sets forth his case clearly and coherently, and with some of his examples, quite entertainingly. If you want some insight into how we think, buy this book.