the catalytic converter is a 'filter' so that harmful waste gasses such as NOx and CO are all catalysed to convert into other less harmful chemical molecules. even unburnt hydrocarbons are converted into other less harmful forms.
most carb cars do not have the cat-con or should i say older models like the datsun 120Ys and such. it's usually after the header. umm, most cars have only one, it's rather expensive to purchase one. the one after the cat-con is usually the mid-box/bullet or whatever it's called, which is used to dampen the resonance from the exhaust system.
if i'm wrong anywhere pls do correct me. don't like giving out wrong info :)