I use the term skills to refer to procedural knowledge and the term understanding to refer to conceptual knowledge.
There is a lot of literature about this. One good starting place is Conceptual and Procedural Knowledge: The Case of Mathematics by James Hiebert (Hillsdale, NJ: Lawrence Erlbaum Associates, 1986). It contains ten chapters by different authors.
Chapter 1 of this book (by James Hiebert and Patricia Lefevre) define procedural knowledge as "made up of two distinct parts. One part is composed of the formal language, or symbol representation system, of mathematics. The other part consists of the algorithms, or rules, for completing mathematical tasks" (p. 6). They define conceptual knowledge as "knowledge that is rich in relationships" (p. 3).
Let me show how I understand this distinction. Let's say the domain is the comparison of fractions. When I say that $\frac{4}{7}>\frac{3}{7}$ because of two fractions with the same denominator, the one with the larger numerator is larger, or when I say that $\frac{3}{7}>\frac{3}{8}$ because of two fractions with the same numerator, the one with the smaller denominator is larger, I am using conceptual knowledge.
But when I say that $\frac{4}{7}<\frac{5}{8}$ because I used the standard procedure of multiplying both sides by the product of the denominators ($7\cdot 8=56$) and got $32<35$, I am using procedural knowledge.