When you look up an exchange rate on a search engine, you are seeing the mid-market rate (also known as the interbank rate). This is the exact midpoint between the price banks are willing to pay for a currency and the price at which they are willing to sell it. It is the 'true' rate of the currency at that exact second.
However, retail consumers and businesses almost never trade at the mid-market rate. Banks, brokers, and currency exchanges are businesses that make money by marking up the exchange rate. The rate they give you will always be worse than the mid-market rate.
Bid, Ask, and the Spread
The foreign exchange market operates on a two-way quote system. The 'Bid' is the price the broker is willing to pay to buy your currency. The 'Ask' (or Offer) is the price at which they will sell you a currency. The difference between the two is called the 'Spread'.
If the EUR/USD mid-market rate is 1.1000, a bank might have a Bid of 1.0950 and an Ask of 1.1050. The 100-pip difference is the spread. If you buy Euros and immediately sell them back without the market moving, you will lose money precisely because of this spread.
Hidden fees in 'Zero Fee' exchanges
Many airport kiosks and credit cards advertise 'Zero Commission' or 'No Foreign Transaction Fees.' While they don't charge a flat $5 fee, they heavily widen the spread. They might sell you currency at an exchange rate that is 3% to 5% worse than the mid-market rate.
For example, if you spend $1,000 USD to buy British Pounds and the mid-market rate is 1.25, you should get £800. If the kiosk gives you a rate of 1.30 (meaning it costs you $1.30 to buy £1), you only receive £769. The 'zero fee' transaction actually cost you over £30 in hidden spread markup.