Let's say a trader has 1,000 USD on their trading account. A regular lot of '1' on MetaTrader 4 is equal to 100,000 currency units. As it is possible to trade mini and even micro lots with Admiral Markets, a deposit this size would allow a trader to open micro lots (0.01 of a single lot or 1,000 currency units) with no leverage put in place. However, as a trader would usually be looking for around 2% return per trade, it could only be equal to 20 USD.
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.