A backsaw has a sturdy spine along the back of the blade that keeps it stiff and straight and it has fine-cutting teeth because it is designed for making precise, straight cuts such as those needed when cutting moldings and trim.
A backsaw is typically used with a miter box, which guides it for cutting 90-degree and 45-degree angles.
Miter boxes can be the inexpensive wooden type shown here or more adjustable, precise metal models.