The pleco has several names, the best one of them 'janitor fish'. Some unflattering names include 'poo eating fish', sucker etc. They are a type of catfish.
The pleco is a hardy fish that can thrive is many conditions, even bad conditions. Other than dwarf plecos (the common ones in fish shop is not dwarf type), plecos can outgrow a home aquarium. However, they grow slowly, so it would still be able to have a good length stay.
Plecos have a wide and low profile and tank skin, which make them suitable for being tankmates of predatory fish to clear food pieces that drop to the bottom of the aquarium where they usually go untouched.
While plecos will eat most foods, it is best to feed them algae pellets or specially formulated sinking pellets also. This is because they sometimes do not get enough to eat on their own.
Plecos does eat fish poo. It breaks them up and eat the edible parts while expelling the rest. In this way, it help to break up big unsightly fish waste and let them go into the filter as particles. (Its own waste is compressed but in thin 'stripes' so they travel to the filter easily too). While poo eaten is a weird concept, it is actually not unusual for animals. Many animals eat poo regulary, these include cute animals like rabbits and mouse etc, not so cute ones like insects such as beetles and flies and even pet animals like dogs (more in the wild where they have to hunt and scavenge for food) and pigs(well they eat anything). Herbivores eat poo because plant matter is hard to digest in one pass. Others eat poo because of the amount proteins it contain. However, do remember that poo eating is not enough especially if there is hardly any uneaten food for them to rasp on. If your pleco go against the glass, take the opportunity to look at its stomach area. If its sunken, trouble is brewing!
One senario is it will die. Another is it it will pass out stringy poo and die. The last senario is it will rasp on your fish (if you have fish about their size or inactive fish esp bottom dwellers) and feed on the fish's slime coat (discus fry feed on the slime coats of their parents but that is not damaging in most cases because of their size). When a pleco feeds on the slime coat of a fish, it takes away the protection that the fish needs and it can lead to bacteria/fungal infections. In any case, this slime coat feeding is also not sufficient and the pleco will also eventually die. The bottomline is to always make sure your plecos get enough food to eat.