Technically, denatured alcohol is ethanol with poison added to it so that it doesn’t have to be taxed and regulated like ethanol intended for human consumption. A lot of alcohol sold as Denatured alcohol is simply methanol. Sometimes denatured alcohol is ethanol with methanol added to it to make it poisonous.
Isopropyl alcohol, sometimes referred to as rubbing alcohol, is one of the many alcohols used for industrial purposes or external medical uses (disinfecting, cleaning, etc.) Most isopropyl alcohol actually has water added to it (usually about 20%-30% by volume) but you can find it nearly pure in some applications.
Small amounts of Methanol or Isopropyl alcohol can be fatal if ingested.
Ethanol is also poisonous, but the amount needed to be fatal is much higher than the fatal amounts of methanol or isopropyl. Accidental deaths from acute alcohol poisoning occur frequently. Typically, those deaths occur when someone consumes an excessive amount of ethanol (liquor) over a short period of time. Young adults in a college environment are common victims. Fortunately, most people will vomit or become unconscious before enough alcohol can be absorbed into the blood to reach fatal levels. But there’s always that special snowflake that drinks an entire bottle of 190 proof Everclear to prove how tough he is. Maybe that’s just Darwin at work?
For many industrial uses of alcohol (solvents, carriers, fuel, cleaners, etc.) it really doesn’t matter which type of alcohol is used. We just want it to be cheap and sometimes we don’t want extra water in the solution. The problem with isopropyl alcohol is that it often has water in it and it can be more expensive than methanol.