"Crochet thread is specially formulated thread usually made from mercerized cotton for crafting decorative crochet items such as doilies or filet crochet." from wikipedia
"Crochet thread is almost always produced from cotton and has a denser pile and smaller diameter than ordinary yarn..." from wikipedia
I had to look this up myself, as I have always referred to the bulky, sweater type as yarn and these type of materials that I have for sale as threads.
Turns out, these are all thread. The mercerized is very fine and will make incredibly beautiful finished products. I actually have a pattern that I am getting ready to start with the mossy green.
The other thread is a little thicker; baby yarn sized; and I have made several products with this type. Baby dresses, toddler dresses, a ladies' vest and fashion belts. This material works and feels like cotton; blocking is very easy!
I hope I have answered your question. Please let me know if I can help you.