Fotoshop is a software created to edit photographs in any imaginable way, fotoshop was developed a while ago by adobe an has been growing ever since, it has a huge active community and constant updates. It features many tools that allow the user to make changes from just a single pixel to the whole global image.
Fotoshop has increased in complexity as well and has become a tool for an above average user, it requieres some expertise, maybe a introduction course, some video tutorials online, take a class and all the way to the designers with a bachellor’s degree. Fotoshop has far exceded it’s competence in widespread use.
Some other photo edition software exist in the world, some even more professional or targeted to a more specific niche, but none of them is as famous as fotoshop.
As it’s to be expected, fotoshop is not free, you need to purchase a one time license and it’s updates. Some online versions have appeared like fotoshop online which allows you to do some similar things, like taking things (parts, objects) from an image and placing them into another image. But it’s interface isn’t identical to fotoshop, only the concept. Some other editors more similar to the original have appeared as well like pixlr, so depending on how familiarized you are with the software you may want to choose between one of those.