The most modern outdoor fireplaces are those that are fueled by propane gas tanks or propane pipes running underground from a source somewhere nearby. This source usually is the same one that allows gas into your home.
In my experience however is that most people use logs as the fuel because the old fireplaces were designed to have wood in them and not gas. Logs are not a very convenient way to run a furnace because of the lack of temperature control and all the smoke that will blow out of it. In some cases you might develop tensions with the neighbors, which can mean that you will need to get rid of your fireplace. Wood burning creates a lot of smoke if done in a poor way. You will need to make sure that no trees are nearby where they can cause a potential fire hazard.
The other source of fuel is coal but is no more popular with product designers and are therefore quite rare in the outdoor garden furniture shops. Coal is also becoming a resource that is slowly going out of stock around the world which makes it much more expensive and a less desirable choice to heat up your garden.
The best way to figure out what to use is the scout your local area for all the options and see what is in abundance and what is not. Different areas will have sources that are less abundant than others, if you can figure out which one is right for your area you could save a lot of money.
