From Wikipedia....

A rant is a purely emotion driven speech or piece of writing that has been sparked by something so emotionally or intellectually jarring that one ignores any notion of making a well researched and calm argument. A rant directly attacks an idea, person or institution, often making few tangible claims and broad, possibly personal, attacks upon the target. While there may be some statistical or concrete information in a rant, the key ideas expressed are what the ‘ranter’ personally feels. A rant may be in defense of an individual, idea or organization but a rant of this manner predominately occurs after said subject has been attacked by another individual or group.

The best examples of rants in popular culture are sports and politics. In sports a rant can come from a coach, player, or fan blogger. Sports provide some of the rawest, most visceral, emotional responses from anyone who cares about the team or event. In politics, a field where one's beliefs are essentially questioned by others, the emotional climate is so volatile that oft-times an individual becomes so incensed that s/he goes on an ill-informed and poorly researched tirade.