Saw Blades in General:
As for the tool they are attaching to,
Saw Blades are offered in different forms, with various densities of teeth (Teeth Per Inch-TPI), made of various materials. Some of them are the following:
They are thin, toothed, bar-shaped ones that perform vertical cutting work. Able to cut wood, MDF, and Steel.
As the name suggests, they are round-shaped and equipped with sharp teeth.
A hacksaw blade should be flexible enough not to be broken when used.
A reciprocating saw blade can be used to cut different materials like wood, hard plastic, or metal. They are not that accurate and their main purpose is mostly demolition.