Forex trading is the trading of currencies from different countries against each other.

Forex is the acronym of Foreign Exchange.

There are a lot of currencies in the world, for example in Europe, they call their currency Euro (EUR) and in the United States, the US Dollar (USD). So, if you are in Los Angeles you would need to buy Euro and simultaneously selling your Dollars (usually your bank is doing this for you by converting). This process is called "going long on the EUR/USD"

Usually, the forex trading can be done through a broker or market maker. As a forex trader you pick a currency pair that you expect to change in value and trade accordingly.

You can place an forex trade on the market which is called 'orders'. With a few clicks, the broker then passes the order along to a partner in the Interbank Market to fill your position (just a simple term that means, fulfilling what you wish to buy and sell). This is all a bit like casino: you are thinking that a certain currency pair will go up, but then it goes down. In that moment, the broker  closes the position on the Interbank Market and credits your account with a loss - or you are lucky and calculated it well, the currency pair goes up and you make a gain and the broker credits your accounts. Believe it or not, but this can all happen within seconds.