This question already has an answer here:
I meant following pages when I say "first pages"
All questions - http://stackoverflow.com/questions
Top questions - http://stackoverflow.com/
Any of the following:
Comments do not bump a question to the top.
Any edit on the question or answer will bump it to the top. Also there are some random questions which might be selected by the system to appear on the top if there hasn't been any activity on them for a long period of time. I have seen many people abusing this though, like adding a space or something insignificant to their question in order to make it appear on the top and attract more attention. Remember that if the question is bad, what it will attract is downvotes instead of answers.