(Thanks to IronyOwl for making the orig. version!)
Scumtells are things that someone does or says that imply that they're scum. Usually the reasoning is that it's bad or not productive for townies to do it, so it can be hard to distinguish signs of being scum from signs of being a noob. This is one of the reasons scum almost always win beginner's mafia games.
Note that scumtells are very very subjective; there's no simple way to distinguish a scumtell from a noobtell from being justified and/or lazy. Whether a justified, non-scummy version of a scumtell should still be called by its normal name varies from person to person. (Someone might describe their voting of someone who voted them as "not an OMGUS," other would say it's "a justified OMGUS" for instance)
In any case, common scumtells include:
Bandwagoning: Voting for whoever has the most votes or seems most likely to be lynched, despite having poor/no reasons or not wanting them lynched previously. It implies that you don't care who gets lynched, you just want someone gone, which is generally the case for scum.
OMGUSing: Short for Oh My God You Suck, by the way. Voting or
otherwise applying pressure to whoever's voting you. It implies that you're really concerned about being pressured/lynched and want them to stop, which is more a scum thing. A townie would, in theory, be more open to the notion that their fellow townie is attempting to scumhunt at them, and less concerned about being found out.
Defending/Chainsawing: Defending just means what it sounds like: saying things to defend another player's actions or words. Chainsawing or chainsaw defending means attacking someone who's attacking another player in an attempt to get them to stop. The idea in both instances is that townies don't know if their fellow players are innocent or not, and hence have no real reason to defend them from legitimate persecution. Scum, on the other hand, know they're on a team and so are prone to not wanting their buddies found out or pressured.
Lurking: Not posting. Especially suspicious in the case of Active Lurking, which is trying to look active while not actually doing anything (posting frequently but with no substance, for instance). The idea in either case is that townies actually care about finding scum, and so should be doing things. Scum don't care, they just want some random guy lynched so they can kill someone at night.
Trying Too Hard To Appear Townie: There's no official name for this one, but being too concerned about your image; specifically avoiding things that might be seen as scumtells. The idea is that scum are very concerned about being found out, and so want to act nonscummy. Townies know they're not scum, so they're less concerned about LOOKING nonscummy and more concerned about finding scum.
Passiveness: This is when a person refuses, simply hasn't or seems reluctant to take a stance, such as lynches. Scum will be concerned about their image, and thus don't want to be seen supporting a town lynch or an attack on a town.
Backtracking: When a person changes their stance on a subject after stating it, such as supporting a lynch and after doing so claiming that they didn't do it. This is very suspicious, and is often a sign of scum wanting to retreat their position so as to be able to save their face.
Lying: Well this should be obvious, right? It happens when someone fakeclaims (says they're a cop/doc, but is just town for example), fabricates evidence and stuff like that.
Rolefishing: Trying to find powerroles (cop and doc in vanilla Mafia), often via questioning.
Resigning yourself to being lynched.
Saying something like "there's no way for me to avoid being lynched so I won't bother trying"
Blackmailing the town, like "You can lynch me, but you'll be sorry when you see I was town all along!"
Voting yourself for any reason.