a click and pawl reel contains a spring thart puts tension on the reel. I dont think any can be adjusted, they only have one setting. Some can be shut off. They do have the advantage of being smooth. Sudden stops due to a bad drag could snap the tippet. THey are fine for most types of fishing.

A disc drag works similar to the drag on a spinning reel. It is an adjustable(sometimes) knob that tightens down on the reel, therefore increasing or decreasing the drag. THey have the advantage of being adjustable(most) and therefore sometimes suitable for larger fish.