Rittle-Johnson & Siegler (1998, pg. 77) distinguish procedural from conceptual knowledge as follows: "We define conceptual knowledge as understanding of the principles that govern the domain and interrelations between pieces of knowledge in the domain (although this knowledge does not need to be explicit). In the literature this type of knowledge is referred to as understanding or principled knowledge. We define procedural as action sequences for solving problems. In the literature this type of knowledge is sometimes referred to as skills, algorithms or strategies."
Rittle-Johnson, B. & Siegler, R. S. (1998) The relation between conceptual and procedural knowledge in learning mathematics. In C. Donlan (Ed.) The Development of Mathematical Skills. East Sussex, UK: Psychology Press.