Stackoverflow is organized by questions: one page for each question asked. The tags are additional details to help classify the question.
For each question, anyone can propose an answer. And anyone with enough reputation can vote on each answer (the arrows next to each answer). You can vote the answer as "useful" (up arrow), or "not useful" (down arrow), or not vote.
The result is that the question-asker will rapidly see the top-voted answers float to the top.
As balpha explained, these features tackle the typical problem of other forums: that you "get lost within a thread, where the 15th post on page 27 of a certain thread usually has nothing to do with the initial post".one