An informal fallacy is a fallacy in which an argument may be valid (i.e., follow the correct form for a logical argument), but the stated premises do not support the conclusion.
As an example, here's an ad hominem attack:
"We should buy stock in Disney."
"You just say that because you look like Mickey."
Or, phrased as an argument:
1. People who look like Mickey Mouse have an irrational affinity for all things Disney.
2. You look like Mickey Mouse.
3. Therefore, your preference for Disney stock is not rational.
This is a valid argument form... but it is probably not sound reasoning, given that it is highly probable that statements 1 and 2 are both incorrect. (If, however, we were to show empirically that both 1 and 2 were true, then this would no longer be an ad hominem, nor would it be a fallacy of any sort.)
Informal fallacies include, but are not limited to: