When you trade forex, you're effectively borrowing the first currency in the pair to buy or sell the second currency. With a US$5-trillion-a-day market, the liquidity is so deep that liquidity providers—the big banks, basically—allow you to trade with leverage. To trade with leverage, you simply set aside the required margin for your trade size. If you're trading 200:1 leverage, for example, you can trade £2,000 in the market while only setting aside £10 in margin in your trading account. For 50:1 leverage, the same trade size would still only require about £40 in margin. This gives you much more exposure, while keeping your capital investment down.
By contrast, if you just traded 20 EUR, a loss would not significantly affect your account balance. It would provide you with the opportunity to learn from your experience and plan your next trade more effectively. With this in mind, limiting the capital you are prepared to risk to 5% of your account balance (or lower) will put you in a better position to continue trading Forex (and improving your technique) over the long term.
Currencies are always traded in pairs, so the "value" of one of the currencies in that pair is relative to the value of the other. This determines how much of country A's currency country B can buy, and vice versa. Establishing this relationship (price) for the global markets is the main function of the foreign exchange market. This also greatly enhances liquidity in all other financial markets, which is key to overall stability.
When trading Forex, you'll see that both 'Bid' and 'Ask' prices are quoted. The bid price is the price at which you can buy the currency, while the ask price is the price at which you can sell it. If you are purchasing a currency in a trade, this is known as a long trade, and the hope is that the currency pair will increase in value, so that you can sell it at a higher price and make a profit on the difference.
Along with being able to access a wide range of financial markets, another benefit of trading CFDs is that a trader can access a much larger portion of those markets, and increase their potential profits as a result. CFD contracts provide leveraged access to the market, meaning a trader can access a much larger portion of the market than what they would be able to purchase outright.