As I write this, the NASDAQ Composite is at 2790.00, down 6.14 points from yesterday. To calculate the percentage, you take 6.14 and divide by yesterday's close of 2796.14 to yield 0.22%.
In your example, if SPY drops from 133.68 to 133.32, you use the difference of -0.36 and divide by the original, i.e. -0.36/133.68 = -0.27%. SPY is an ETF which you can invest in that tracks the S&P 500 index. Ideally, the index would have dropped the same percentage as SPY, but the points would be different (~10x higher).
To answer your question about how one qualifies a point, it completely depends on the index being discussed. For example, the S&P 500 is a market-capitalization weighted index of the common stock of 500 large-cap US public companies. It is as if you owned every share of each of the 500 companies, then divide by some large constant to create a number that's easily understood mentally (i.e. 1330). The NASDAQ Composite used the same methodology but includes practically all stocks listed on the NASDAQ.
Meanwhile, the Dow Jones Industrial Average is a price-weighted index of 30 large-cap companies. It's final value is modified using a divisor known as the Dow Divisor, which accounts for stock splits and similar events that have occurred since a stock has joined the index.
Thus, points when referring to an index do not typically represent dollars. Rather, they serve as a quantitative measure of how the market is doing based on the performance of the index constituents. ETFs like SPY add a layer of abstraction by creating an investible vehicle that ideally tracks the value of the underlying index directly.
Finally, neither price nor index value is related to volume. Volume is a raw measurement of the total number of shares traded for a given stock or the aggregate for a given exchange.
Hope this helps!
