Most conferences are supposed to have a selection process (in most cases a "peer review" process) in which all submissions are sent to multiple reviewers who are experts in that area, and only papers which are deemed "acceptable" by a majority of the reviewers are published. Usually this means that the paper should be relevant to the conference, should have results that are worth publishing, and should have something new - something that hasn't been done before.

All of this is supposed to happen before the paper is accepted for publication. The proceedings of top conferences in most fields are, in my opinion, more important than the actual conference itself.