Plecos (short for Plecostomus) are large scavenging catfish with a horny covering over their bodies for protection.
There are other species of pleco (several genuses, in fact, but all are of family Loricariidae), some of which have wild coloration, but which are rarer and usually cost much more than H. plecostomus. All have the same basic behavior; as a group they are also known as "sucker fish" for their ability to attach to all sorts of objects, even glass, by aid of specialized mouth. They do usually do a pretty good job of cleaning algae off of objects, but some individuals are not as good as others for this.
They are not for every tank... did you know that these guys can grow a wopping 2 or 3 feet? They can be started out in medium sized tanks (40 gallons and up) when they are small, but you should know that they are capable of growing fast, so sometimes its better just to start them off in a really big tank. They are also relatively big ammonia producers, another reason why they should not be kept in the smaller tank. They're actually wonderful bottom feeders for the large aquarium with big semi-aggressive fish such as S.A. cichlids, as they can usually hold their own with their bony plate protecting them from harrassment.
Plecos are primarily herbivorous, meaning they eat a lot of plant matter, but they will take most scraps offered and adapt well to the tank if they get over the initial adjustment phase (I have found that they do shock with sudden changes, but many other fish are the same way).
To supplement their diet with the proper nutrition (no, they can't just live off of algae and fish waste, this is a myth; they can starve to death), add occasional algae wafers or catfish tablets once a week.
In addition, they also enjoy fresh plant matter, such as the occasional spinach leaf, zuccini slice, or a bit of cucumber--even a tiny slice of orange or melon! These can be kept on a "veggie clip", which anchors the food to the aquarium wall.
They sometimes also supplement it themselves by gnawing on African driftwood or eating decaying plant matter (they usually leave live plants alone unless they are unhealthy to begin with). One annoying behavior that I've heard they can have is that they will sometimes anchor themselves to the body of large fish, like a discus, angelfish or goldfish, and try to eat the natural slime on their skin. I have personally not seen this happen with my old angels, but I've only owned one pleco.