A screener (SCR) is an advance screening of a film or television series sent to critics, awards voters, video stores (for their manager and employees), and other film industry professionals, including producers and distributors. Source
From what I see in torrent sites, screeners are often released in DVD format.
Why won't film makers send screeners in Blu-ray discs, at least for a better visual on the film since Blu-ray video has a higher resolution than DVDs? Also IMO, the Blu-ray disk may have better protections to prevent the movie from being copied or ripped.