Forex trading is usually done through a broker. As a Forex trader you can choose a currency pair that you want to change in value and place a trade accordingly. 

Forex trades can be placed through a broker. Orders can be placed with just a few clicks and the broker then passes the order along to a partner in the Interbank Market to fill your position.
When you close your trade, the broker closes the position on the Interbank Market and credits your account with the loss or gain. This can all happen literally within a few seconds.

Forex trades (trading on board internet platforms) are non-delivery trades: currencies are not physically traded, but rather there are currency contracts which are agreed upon and performed. Both parties to such contracts (the trader and trading platform) undertake to fulfill their obligations: one side undertakes to sell the amount specified, and the other under takes to buy it. Thus, the contract ends by offsetting it against an opposite position, resulting in the profit and loss of the parties involved.