Peel off the transfer film, stick it on something, remove the film, and voila!
Complex shapes usually need a clear sticker transfer "tape", which is the grid that you see over the stickers
As long as you can vectorize the image, you can Vinyl cut it. It's especially easy to cut black and white images, as well as images without gradients. Ability icons from games and logos are the easiest to vinyl cut.
Peels off easily, without mess (at most some weak glue residue that can be washed off). Vinyl is made to be easily attachable and easily removeable. Stickers are no different.