Ignore MMZero 's future advice. Register for Free. It's years like this where both the seasoned investor and the newbie discover their tolerance for risk. Of course, the other indices have their shortcomings as well. Hardly a good indicator of the market as a whole.
Peak refers to the highest level the index reaches before it begins to fall.
The peak is the beginning of a bear market. Trough is the lowest level of the index and marks the end of the bear market. Bear Market is a time of falling stock prices. On the chart, it's the period between the Peak and Trough. If you held on from the darkest hour in the last three bears, after five years you would have nearly doubled your money twice and nearly tripled it once, a compelling argument to stick out this current storm.