Costs
Swap
Swap is the interest differential between two currencies in a pair, charged or credited when a position is held overnight.
Spread
The spread is the difference between the bid and ask price. It represents the cost of trading and is how most brokers make money.
Slippage
Slippage is the difference between the expected price of a trade and the price at which it actually executes. Common during high volatility or low liquidity.
Rollover / Swap
Rollover or swap is the interest earned or paid for holding a forex position overnight. It is based on the interest rate differential between the two currencies.
Commission
Commission is a fee charged by brokers for executing trades. It can be a flat rate per lot, a percentage of trade value, or built into the spread.