A Post-Hoc Fallacy is also known as a post hoc, ergo propter hoc, which is Latin for "after it, therefore because of it." It is an assumption that, since two events occured, one event was caused by the other event. It ends up as a conclusion that makes sense but doesn't have the evidence necessary to prove it. Sometimes these assumptions make sense: for example, I could say that since my sister was the last person to use the computer, and then the computer broke, my sister broke the computer. It seems logical that the last person to use the computer was the one who broke it, even though there is no clear-cut evidence to back it up.
Lawyers also use post-hoc fallacies to persuade the jury to believe that someone is guilty. They may say that the defendant was in the house at the time of the victim's murder, so therefore the defendant must have killed the victim. Again, this argument is very persuasive but cannot be proven true.
I found a great example of a ridiculous post-hoc fallacy in an episode of "Friends." Joey broke his refrigerator, but he doesn't have enough money to buy a new one, so he tries to blame all of his friends so that they willl pay for a new fridge. But there are definitely some holes in his thinking... check it out: