Loose gold leaf is individual sheets, typically 3.25 inches square, separated by tissue paper. Transfer leaf is gold that has been adhered to thin tissue backing. The difference is not small.
Loose leaf requires more technique. It is more delicate—a breath can blow it away. You must use a gilding brush (typically squirrel hair, very soft) to apply it. The process is slower, more deliberate, and it demands precision. But loose leaf gives you complete control. You can position it exactly, shape it, layer it, cut it, manipulate it on the surface. Historically, this was the standard method because it allowed for the highest level of finish quality.
Transfer leaf is easier to handle. The tissue backing protects the leaf and makes it less susceptible to air currents. You can press the tissue down onto the sized surface and the leaf transfers to the ground. It is faster. But you lose some control. The leaf is already positioned; you cannot adjust it precisely the way you can with loose leaf. For high-precision work, loose leaf remains the standard. For faster production or less technical applications, transfer leaf is practical.