Chiller is basicly a second Sci Fi Channel. It's to Sci Fi like Boomerang is to Cartoon Network. It basicly airs older shows that no longer air on Sci Fi (With a bit more of a monster/horror theme it them) and thats it.
At the beginning of July it started it's own Animonday at 9PM. Airing the shows Sci Fi use to air. In a sense Animonday is now a four hour block.
Chiller is NBC-Universal's suspense/horror oriented channel. It is available on some extra extended digital cable and satellite packages, but not nearly as widely seen as Syfy, also from NBC-Universal.