the difference between a non-woven & a woven interfacing is:
non woven is like a paper- woven is like a lightweight fabric
i use woven interfacing in my t=shirt quilts- it is softer & doesn't tear as easily as the non-wovens do
you do not want (paper backed fusable) like heat n bond-or wonderunder---you want to ask for lightweight fusable interfacing- pellon seems to be the one most places like joannes carries- it is white, thin (see through) with a bumpy side (the side with the fusable on it) and a smooth side- the bumpy side goes on the back of the t-shirt decals, you press it on- it stablizes the shirts so they don't stretch out of shape while you are sewing. it is the same stuff found in the cuffs/collars of shirts (except those sometimes are a heavier weight) or along a placket (where the buttons/button holes are on the shirt)...just adds some stability- and stays in- does not need to be removed.