Plastisol transfers are designs printed by a screen printer onto a special backing sheet and posted to you. You then simply press the transfers onto your garments and the result is virtually indistinguishable from a screen printed garment.
Obviously, the screen printing outfit has set up charges to consider, so it's only a cost-effective solution if you are ordering, say, 50 sheets or more of the same design in the same colour(s). Designs can be 'ganged' onto the sheets though. Also, it costs more if more colours are used.
Many plastisol suppliers will provide you with free samples to try out in your press, on various materials.