Q: Are skew lines two lines that do not intersect and are not parallel? ¶
Q: Are skew lines to apply the formula for the volume of a tetrahedron in terms of its four vertices? ¶
Q: Are skew lines the pair of lines through opposite edges of a regular tetrahedron? ¶
Q: Are skew lines the "usual" case? ¶
A: Yes, and parallel or intersecting lines are special cases.
Q: Are skew lines a set of lines in which all pairs are skew? ¶
Q: Are skew lines defined by two points that it passes through? ¶
A: Yes, and then these four points must not be coplanar, so they must be the vertices of a tetrahedron of nonzero volume.