What are the different "types" or duty ratings of ladders?

Type IA ladders are industrial rated, with a working load of 300 lbs. maximum. They are considered to be rated for "extra heavy duty".

Type I ladders are also industrial rated, but with a working load of 250 lbs. maximum.  They are considered to be rated for "heavy duty".

Type II ladders are commercial rated, with a working load of 225 lbs. maximum. They are considered to be rated for "medium duty".

Type III ladders are household rated, with a working load of 200 lbs. maximum. They are considered to be rated for "light duty".