Some domain companies that offer you cheap or free domain names actually register the domain name in their own business name, then "let" you use it.
This means that if your business takes off, they own the name, and can begin charging you for it, or sell it to you for an exorbitant fee.
If you are after a name for a bit of fun, there is no problem, if you are serious about business then you need to make sure you register the name yourself.