All catalytic converters function in basically the same way: exhaust gases superheat a substrate containing various elements, which causes subsequent harmful emission to convert to a more inert form. OBD-compliant cats are simply an improvement in design over older units.

Converter Components

A catalytic converter contains a matrix of material through which exhaust gases flow. When these gases compress on the substrate surface, they drastically increase in temperature and initiate "light-off." Once light-off temperature is reached, the cat can begin converting toxic gases to other forms, as determined by the material used in its construction.

OBD Definition

On-Board Diagnostics (OBD) is a catch-all term used to define any computerized system that monitors driveline condition. Cars made since 1996 carry OBD-II, which is a highly sophisticated update to the original OBD-I.

Pre-OBD Cats

Prior to computer controls, cars were often equipped with lead pellet-type cats. By the standards of the time, lead-pellet cats did an acceptable job but were quickly replaced by superior designs.

OBD-I Cats

The first computer-controlled cars used cats with a stainless-steel matrix and a more advanced wash-coat, enabling them to be higher-flowing and more efficient.

OBD-II Compliant Cats

Post-1996, cats must meet a strict set of government guidelines that dictate the type of materials used, the levels of emissions allowed and the rate at which catalyzation occurs.

Source:

What is OBD-II?

ODB II Catalytic Converter Description

OBD Codes Forum

Resource:

OBD-II Scanner Review (Video)