According to the Fallacy Files, a slippery slope argument is an example of an informal fallacy:
An informal fallacy is one that is not formal, that is, it is a type of fallacy in which the content of the argument is relevant to its fallaciousness, or which is fallacious for epistemological, dialectical, or pragmatic reasons. Typically, informal fallacies occur in non-deductive reasoning, which relies on content as well as form for cogency. Also, because content is important in informal fallacies, there are arguments with the form of the fallacy which are cogent. For this reason, when forms for informal fallacies are given, this is for identification purposes only, that is, one cannot tell from the form alone that an instance is fallacious. Rather, the forms will help to differentiate between distinct types of informal fallacy.
To put it another way, just because someone uses an argument in the form of a slippery slope does not mean that their argument is invalid or fallacious. It's perfectly possible to construct a slippery slope argument that works and is persuasive:
Don't light that cigarette or the next thing you know you'll have lung cancer!
It works because smoking is addictive (so once you start it's difficult to stop) and does cause various types of cancer. Obviously not all people who take up smoking will get lung cancer and some people with lung cancer have never smoked. But that's not what the argument is claiming. Rather it's attempting to persuade the hearer that making a certain choice (taking up smoking) could have unintended consequences outside of their control (getting cancer).
A similar argument is fallacious:
Don't go to that dance or the next thing you know you'll be pregnant!
This doesn't work (even if the hearer is female) because there are many things a person can do to avoid getting pregnant at a dance.1 A dancer might say "No" to a potential liaison or use contraception, for instance. The argument doesn't hold up if there are subsequent decisions that can be made to prevent an a person from slipping to the (presumably) undesired bottom of the slope.
There's a closely related argument that's sometimes called the semantic slippery slope. Rather than being of the form "X leads to Y, which leads to Z", it goes "X is like Y, which is similar to Z". This version depends on the vagueness of the lines between X and Y and Z. We see this sort of reasoning a lot in the abortion debate:
Killing a baby after birth is wrong, therefore killing a fetus at 9 months is wrong, therefore killing a fetus at 8 months wrong, ...
A fetus is just a mass of cells in a woman's body so removing them has the same ethical complications as removing [some other mass of cells].
Neither of these arguments are cogent on their own since reasonable people can draw clear lines between one situation and another. Is a fetus a part of the woman's body or is it an independent human? The lines are fuzzy, so it sort of depends on where you start. Neither argument is particularly persuasive for people who make different assumptions or have different ethical priorities.
A semantic slippery slope argument could be cogent, but it's also a red flag that there might be something fishy going on under the surface.
1. The old joke: A Baptist minister once warned young people against sex because it leads to dancing.