Depending on the modeling technique that is being applied you may or may not use curves in the construction of some surfaces.

If curves will be used there are many options that can be applied.
Sketches
Studio Splines

If a fully numerically parametric method is the option to be used then the obvious choice is sketch. Sketches can be used as frames, sections and guides. Sketches allow expressions to drive the design. Meaning I can give a numerical value to the inputs. By changing said values the parametric class a surfaces associated to the sketches will in theory update seamlessly. Of course the seamless updates can only occur if the model was built in a fashion that allows easy updates.

Studio splines are another method of curve creation. These curves are exceptionally powerful in that the way they are constructed can easily be changed. As well as how the curve is constrained to adjacent data. They also offer a G3 constraints for "Flow".

The following posts will describe in further detail the uses of each as well as there limitations.