When you make a trade, you use Trading Leverage. In fact, the company gives you a special loan, which according to certain rules it takes from the bank or from its own funds. If you have $100 in your account and you buy 1000 euros on the market (which are worth $1,200) or 0.1 bitcoin, which are worth $1,100 (as of January 2018) – you do so with leverage. At the same time you pay the difference between the Central Bank refinancing rates or in the case of Cryptocurrency – some small %. Let’s take a classic example:
Suppose Europe’s rate is 4.0% and America’s is 3.5%. Suppose you have an open Sell position to buy 100,000 euros for EUR/USD. You must borrow them at 4.0% per annum. After selling euros, you buy dollars, which you give back at 3.5% per annum. In total, your transaction costs are: (4.0-3.5%) per annum, or the same for a EUR/USD rate of 0.9000: $675 per year, or $1.85 per day, or 0.18 points if trading at 1:100.
The amount credited / debited as a fee for transferring a position is called storage or swap.
Note: Swap is charged/accrued triple for a Wednesday night position rollover from Wednesday to Thursday. This is due to the fact that the position opened on Wednesday has a value date of Friday. When a position is rolled over from Wednesday night to Thursday night, the value date should increase not by 1 day, but by 3 days and becomes a Monday. Therefore, the swap from Wednesday to Thursday is charged/accrued in triplicate.
The current swaps for trading instruments, you can find here.