A slippery slope is when you appeal to emotion rather than reason to imply that a relatively insignificant event will lead to a whole series of other events in sequence (which are inadequately explained), culminating in a more dramatic or significant event.
Yeah, it's an appeal to emotion based on a prediction of a chain of negative indirect/unintended results. Slippery Slopes are the abused bastard children of the law of unintended consequences and murphy's law
There are two separate things here.
Q: Is the slippery slope always a fallacy?
A: Yes. It is by definition bad logic. Remember, pointing out a demonstrable cause/effect relationship is not a slippery slope. "If you keep running with scissors you might hurt yourself" is not a slippery slope. "If you throw a match at that petrol it will explode" is not a slippery slope. As people have mentioned, you can also connect these statements into coherent chains of causality without it being a slippery slope.
A slippery slope is when you appeal to emotion rather than reason to imply that a relatively insignificant event will lead to a whole series of other events in sequence (which are inadequately explained), culminating in a more dramatic or significant event. "If you let your children play video games, next thing you know they'll be taking drugs and joining gangs" would be a slippery slope. There is no causal relationship between these two things, it's just playing on a generalized fear that a particular course of action will lead to a whole sequence of "things going to hell" for poorly substantiated reasons.
Q: Is the slippery slope fallacy always untrue?
A: No. Being a fallacy has no bearing on whether a statement is actually going to be true. The fact that the above slippery slope is a slippery slope doesn't mean that if I let my children play games they're guaranteed never to do drugs or join a gang. The fallacious nature of the argument is purely a comment on the assumption of causality.
It's even possible that the fallacious person might turn out to be correct and there is a cause/effect relationship which was not originally obvious, but it would still have been a fallacy to assume that based on the logic being presented.
This is helpful I understand it better now.