Does this tack strip actually have the tacks embedded in the cardboard?

No there is no tacks embedded in the cardboard itself. This is just the card board. I used it to upholster a couple of cornices. This is used to fold over the fabric and make a clean seem. Done by placing tack strip over the wrong side of the fabric and stappling above the tack strip (I used an electric stapler). This created a nice seem line once I turned the fabric over on its right side.

are there actual tacks in this product or is in just a strip to place tacks in?

No tacks - just a cushion so to speak to place tacks or staples into. This gives the fabric a crisp edge when you fold it over top, then you staple thru the fabric and tack strip. Worked quite well for me and I'm a real novice!

Do you have to sew it on?

No, you use it to stiffen the fabric, especially around the corners (like in a chair) and then it is stapled.

what kind of /or size tacks would you use

The tack size depend on the wood you may be shooting into. If softer, like Birch, depending on the thickness of the fabric, 3/8 should do, thicker fabric 1/2 inch.Maple or Oak using any size staple, I had to hit with a hammer afterwards. I finally went to an electric powershot gun and used heavy duty staples.