D3 lines are rendered using linear interpolation by default. This means that the line connecting a series of points will trace a direct path from one node to another:
The curved line is rendered with the cardinal interpolator, one of several options D3 provides. The curvature of the line is called a spline. Splines are tools used by architects to draw curved lines, and the term was adopted by mathematicians to describe smooth, piecewise polynomial approximation:
Splines are curves, which are usually required to be continuous and smooth… [The] join points are called knots. Splines with few knots are generally smoother than splines with many knots; however, increasing the number of knots usually increases the fit of the spline function to the data. Knots give the curve freedom to bend to more closely follow the data. —Wikipedia