Neil Fein asked: How is Stack Exchange different from a web forum?
Rory Alsop answered: The aim is to provide specific answers to specific questions that will be valuable to future visitors who have the same question - to be a site providing 'expert' advice. Web forums inspire subjective discussion (explicitly discouraged here - except in chat) - which doesn't necessarily encourage well thought out, intelligent, 'expert' answers
Gary.Ray answered: Of course, the primary difference is the expectation that questions asked will have an objectively correct answer. I know there is a range of what is allowed as questions, but the absence of the typical rants, shopping and pure opinion/speculation questions is what makes this site a gem.
Gary.Ray continued: The voting/reputation action tends to weed out the trolls and flamers, which is fantastic. The best feature is that the best questions and answers bubble up to the top.
freiheit answered: Forums tend to be very chatty. Post a question, get some answers, some comments, some more answers. A later person with the same question basically has to read the whole thread, because the best answer is hidden as the 4th from last in the thread. SE is very focused on questions and answers. Not chatty, not too talky. The best answer gets voted and/or accepted and shows up first for future visitors with the same question.
Unsliced answered: It's Q-and-A first and foremost, it's about providing answers that will continue to be true, to shy away from chatty topics and to allow someone to give THE answer to a question, there for all to see and be found.
ʍǝɥʇɐɯ answered: 'Bikes.se' should be the same as 'overflow' - best answers at the top. Really I see it as the site being a body of expert knowledge with 'wisdom of crowds' making it so, rather than on a forum where the first answers may not be the right ones.