Most currency exchange brokers offering accounts to retail traders operate in one of 2 ways. Much more likely, you will be having a look at either an ECN broker or a market maker.

ECN currency exchange brokers use the Electronic Communication Network, a worldwide online marketplace that caters for many different types of trader from retail to the massive banks and market makers. The spread on the ECN is tiny, sometimes almost non existent, so brokers using this network will usually either add 2 pips to the genuine spread or charge commission or costs per deal. You can often improve costs from an ECN broker but take a detailed look at their fee structure and consider what it would mean to you on a standard deal. Slippage is not most of an issue , either for scalping or at times of forex news reports. On the downside, the variable spread can imply more uncertainty when setting stop losses and limit orders. ECN brokers also tend to offer fewer charts and can have a less user friendly dealing platform because they are not in particular trying to attract newbies. They generally tend to assume that you know what you do and have a paid subscription to do your technical analysis some place else.