Just like stocks, you can trade currency based on what you think its value is (or where it's headed). But the big difference with forex is that you can trade up or down just as easily. If you think a currency will increase in value, you can buy it. If you think it will decrease, you can sell it. With a market this large, finding a buyer when you're selling and a seller when you're buying is much easier than in other markets. Maybe you hear on the news that China is devaluing its currency to draw more foreign business into its country. If you think that trend will continue, you could make a forex trade by selling the Chinese currency against another currency, say, the US dollar. The more the Chinese currency devalues against the US dollar, the higher your profits. If the Chinese currency increases in value while you have your sell position open, then your losses increase and you want to get out of the trade.
Since currencies are traded in pairs, it’s impractical and not very useful to draw a pure USD currency chart. Instead we have the option of drawing (or rather having the software plot for us) a chart of the USDJPY pair, or the AUDUSD pair, since it is only possible to quote a currency in terms of another. On the other hand, there are some forex charts that take weighted average of such currency pairs to derive an overall index for a currency. The famous USD index, is a good example.