Travel rewards credit cards can be confusing. We all know people who say they have flown around the world on free frequent flyer miles from their credit cards, but if you would like to do it yourself you may not know where to start.

To begin with, you must know the difference between the types of frequent flyer and travel rewards credit cards offered. Here is a primer on the two main types of cards:

Miles that are not really miles

Some cards say they give you miles as a reward, but the “miles” are not really miles. They are, in many cases, simply worth a certain amount of money off a travel charge on your credit card. This is the case with most cards that are not co-branded to a specific airline.

Here’s how it works: you get a certain number of miles per dollar spent (Capital One gives double miles, or two miles per dollar spent) and are free to redeem them however you wish. You can cash in your miles for actual cash, a statement credit, gift cards, merchandise, or plane tickets. The miles have a value of 1 cent each, no matter how you redeem them. That means you can get a $25 gift card for 25,000 miles, $50 in cash for 50,000 miles, or a $200 plane ticket for 200,000 miles.

The advantage to this is the flexibility. You can use those miles any way you like, and if you choose to spend them on air travel, you can fly any airline with no blackout dates or restrictions.

Miles that are truly miles – with a catch

Then there are other cards, often co-branded with a certain airline but not always, that truly do offer frequent flyer miles as a reward. A good example is American Express, which gives some of the most valuable rewards in the industry. The Gold Delta SkyMiles card gives miles to cardholders – they even have a sign-on bonus of 30,000 bonus miles for new cardholders right now, when they spend a certain amount in the first three months they have the card.

The catch is, these miles really are miles. They cannot be redeemed for other things, like gift cards or cash back. And they can only be used on Delta or one of Delta’s air alliance partners. Delta is a member of SkyTeam, so customers can transfer their miles to any SkyTeam airline for travel. There are sometimes blackout dates, seat restrictions, or other guidelines that travelers must follow when booking reward travel with this type of card.

The reason many people prefer these “true miles” cards is that the value is higher, so you can get more bang for your buck. For instance: a flight from Los Angeles to New York might cost 40,000 miles. This will be the same whether the flight is during peak times, or whether you buy the ticket six weeks in advance or two days before. The cash value of that flight will fluctuate, but the miles value stays the same. Therefore, 40,000 miles can be worth much more than $400 – the one cent per mile value of other travel cards.

They type of travel rewards card you choose will depend on your personal preference and travel habits. But knowing the difference is the first step in finding the perfect card.