A bi fold or tri fold is a booster that's a large square that you fold up and stuff into a pocket or nappy. It's usually made up of two layers overlocked together.
The advantage of it is that it dries quickly because it can flatten out compared to a booster that was made from the same amount of fabric layers.

The other things I'm sure that I've read somewhere that the more dense or heavy the fabric, then the more absorbant it is. I also think I've read that cotton is added to strengthen it a bit but I could be wrong there.