Segments between two sharp (corner) nodes are called line segments.
Segments between sharp and smooth or between two smooth nodes and are called curve segments.
Curve segments can be of two types: PostScript (PS, cubic Bézier) curves and TrueType (TT, quadratic) curves. A single contour cannot mix different kinds of segments in final font formats—indeed, it is not acceptable to even have both kinds of outlines in a single font. However, FontLab allows you to do so for design purposes, and can convert at output time.