You might have seen websites related to creating trainer cards for pokemon. A Pokemon trainer card maker is a web based program that allows you to make trainer cards. Some of these websites are free and allow you to create trainer cards from different Pokemon games. They use unique backgrounds made by fans and players. It is also possible to create trainer cards without using a Pokemon trainer card maker since most of these trainer cards are pixelized and it's possible to create them using paint or photoshop.
An example of one made without a Pokemon trainer card maker you can find here:
You should avoid websites that want you to pay to use the Pokemon trainer card maker because there are enough free websites that allow you to use them. Sites that ask for money are just trying to cheat you. But it's still wise to scan any downloads from sites that claim to have Pokemon training card makers because you would not want to download a virus by mistake.