It can vary from the finest of threads, to the bulkiest of cord. Choosing what is right for your project is important. You may want to consider how soft one feels for a babies blanket, or an absorbent cotton if used around water. The PLY of the yarn is simply the number of spun threads that are twisted together to make a strand of thread or yarn. It has nothing to do with the weight or thickness of the strand.
Hooks come in a range of sizes and materials.
Picking the right hook size is important. The shaft of the hook determines the size of the stitch. Instructions will give you the size needed to make that particular project. However, not everyone crochets exactly the same. Sometimes we have to increase or decrease the size hook we are using in order to achieve the proper gauge indicated.