Heat transfer paper is the transfer paper used in heat transfer printing. An image is printed in reverse/mirror onto the paper; a heat source (heat press or hot iron) is then used to transfer the image from the paper to the item being printed.
Heat transfer paper is sold for both inkjet and laser printers (most laser paper work fine on modern photocopiers as it’s the same printing method). You also get a transfer paper that is used for screen printing.
The most common type of transfer paper for inkjet printers is the textile paper most popularly used for t-shirts. You get one for light garments and one for dark garments. The light garment one has no background while the dark garment paper essentially has an opaque white background.
Laser printing has a larger variety of heat transfer papers; you get the textile papers like you get for inkjet. And you also get a “soft touch” textile paper which does not leave the plasticky background commonly found in inkjet printing. You also get a “hard surfaces” paper which is used to print onto ceramics such as tiles, on wood as well as on other surfaces such as leather which is the paper used to print onto balls.