I made a small diagram to show the order the games came out:
School Days is the main & first game in the series. It was remade in School Days HQ with updated graphics and additional uncensored scenes.
Summer Days takes place during the same time period, but in an alternate universe. The story is told from the perspective of Setsuna Kiyoura. It was remade as Shiny Days and an English translation of this has been announced recently. The original Shiny Days came bundled with bonus games Ahoge Battle and Strip Battle Days which haven't been confirmed/unconfirmed by JASTUSA yet whether they will be in the English version or not.
Cross Days is a story that takes place at the same time as School Days, but with a different cast. There is no English version yet, although there was a trial CD at one stage.
Island Days is a VN/Tower Defence Game, where weapons picked up in the main story line are used to defend yourself during the tower-defence stage. It seems to just be a spin-off unplaced in time.
As of writing, only School days has an animated version, but Summer Days and Cross Days have small manga adaptions - An anthology and a two-volume series respectively. There are light novels of all three games.
There doesn't seem to be much secondary merchandise for Island Days.