What is the difference between suck and suckle?

The suckle is the earlier pattern for babies. There is a back and forth movement of the tongue, and the tongue stays within the baby's mouth. There is not a very strong lip seal.

Around 3-6 months the suck should develop. There will be a stronger lip seal and the tongue will have an up and down movement.

Both of these are different from nonnutritive sucking, as they might to do a finger or a toy, etc.