In the previous USD/JPY example, between 2005 and 2006 the U.S. Federal Reserve was aggressively raising interest rates from 2.25% in January to 4.25%, an increase of 200 basis points. During that same time, the Bank of Japan sat on its hands and left interest rates at zero. Therefore, the spread between U.S. and Japanese interest rates grew from 2.25% (2.25% - 0%) to 4.25% (4.25% - 0%). This is what we call an expanding interest rate spread.
Currencies are traded on the Foreign Exchange market, also known as Forex. This is a decentralized market that spans the globe and is considered the largest by trading volume and the most liquid worldwide. Exchange rates fluctuate continuously due to the ever changing market forces of supply and demand. Forex traders buy a currency pair if they think the exchange rate will rise and sell it if they think the opposite will happen. The Forex market remains open around the world for 24 hours a day with the exception of weekends.