Can someone explain to me the difference between fusible web and interfacing and fusible fleece and steam a seam and wunder under and all that....stuff?

Can't help with fusible web as I have never used it. Have use Wonder Under and love it. I tried a Walmart brand of same and it was a big disappointment.

Steam a Seam and Wonder Under work pretty much the same way. The bumpy side of this stuff is usually placed against the wrong side of the fabric and pressed. The paper backing is peeled away and the melted bumps stick to the fabric, allowing you to fuse it to other fabric. These products come in a variety of weights and stiffnesses.

Fusible batting is ironed to the wrong side of fabric. Again the bumpy or rough side is against the wrong side of the fabric. Then the fabric is pressed and the batting fuses to the fabric.

Fusible web is a product that comes on a roll in widths of about a half inch. A section of that is placed between two sections (or pieces) of fabric on the wron sides of both and is then pressed, fusing the sections together. Such as for an instant hem. There is another product on the market that is called Misty Fuse that is in larger pieces of this stuff.

Ok so if I want to applique a fabric letter onto a pillow cover that Ive made and I want the letter to stick in place before I sew around the edges....wunder under?

and thanks for your help!