Heat transfer is similar to silk-screening. The quality of the print is similar (the print will not crack or fade) but the physical process is different.  Heat transfer designs are first printed on or cut from a carrier sheet. That design is then placed onto the garment and a press of about 390 degrees clamps down melting the design onto the item. During the British Columbia Provincial Scouts Jamboree of 2003 we did over 1,500 t-shirts with this method and had no issues with quality.