What role do conferences play, for astronomers?

At least some conference proceedings find their way into ADS, and it seems at least some authors of conference papers get them up on astro-ph.

Are conference papers (peer) reviewed? In terms of acceptability, to professionals, how do they compare with papers published in peer-reviewed journals?

Are conference proceedings published?

And what are posters?

To what extent do conference organisers take the trouble to make agendas, posters, papers, etc available on the internet?