Formal vs. Informal Arguments
The difference between a formal and an informal argument is in the burden of proof. A formal argument clearly states the claim or position it argues and presents a well-developed chain of evidence leading to a reasonable conclusion supporting the claim. The chain of evidence itself may include a wide variety of elements ranging from personal experience to statistical data and expert testimony.
Informal arguments contain little or no supportive evidence. "I did the dishes last night" may be all that's necessary to encourage your roommate to do them tonight but it's hardly an argument designed to convince or persuade. Its primary purpose is merely to assert, or point something out, nothing more.
Informal arguments are the stock-in-trade of radio and TV talk-shows, op-ed pages and letters to the editor. Generally speaking, they're used to instigate discussion among individuals holding different opinions. Quite often they are used to provoke a confrontation between those who flat-out disagree with each other (e.g., The O'Reilly Factor and The Jerry Springer Show). Seldom do they end in a consensus of opinion or a reasonable conclusion.