Plecos have small rough bumps inside their mouths that allow them to remove algea from surfaces, but no "teeth". Furthermore, they're herbivores and don't eat meat as part of their diet. They have no interest whatsoever in "eating" a fish. You may see one at a pet store that is "stuck" on a dead fish, but the pleco did not kill it, suck it to death, nor is it trying to eat it. They will occasionally get stuck on fish pellets too, upon which they flip upside down to get the pellet to float out of their mouth. When they suck on driftwood they are extracting the sugars from the wood as a source of nutrience. They do not actually "eat" the wood.
Like I said, their fins are spiney just like any catfish, and they can use them as a defense mechanism to "stab" something if they feel threatened (such as when they are being removed from a tank), but they are too slow and clumsy to use these in any event other than self-defense. They aren't aggressive at all and usually retreat in the event that another fish is picking on them. Larger plecos (8-10" range) will defend territory or chase a fish that is bullying them. I've seen mine headbutt two fish that are fighting from time to time, but never harm them.
On the other hand, Chinese Algae Eaters (which novice fishkeepers sometimes confuse with plecos because of their name) are aggressive and territorial and do kill other fish on occasion.
Do some research an reading. There are forums dedicated specifically to plecos. They're interesting fish and fun to keep, but know what you get before you buy it. Plecos can vary vastly in size, coloration and diet specifications depeding on the kind you buy. But, for the most part they're pretty low maintenance and help make your tank a little more low maintence as well.