Unless the creatures are hiding, the locations of all creatures are known to all other creatures.
Bright light means that everything in the room is visible. provided it's not concealed in some other way.
Dim light provides light obscuration which provides partial concealment against folks without low light vision. That's a -2 penalty to attack rolls. (PHB 281, RC 220)
Darkness is considered totally obscured. This provides total concealment against folks without darkvision. That's a -5 penalty to attack rolls. (PHB 281, RC 220). The target is not visible so Opportunity attacks are not provoked. Creatures with total concealment can also make stealth checks to hide when they move.
There is another type of obscurity which could be used depending on the light. That's Heavily Obscured. There really isn't much mechanical guidance on how to use this with light, but it's a factor if dense fog is present, and I'd argue more than a few squares of dim light between the target and the attacker. Creatures in heavily obscured squares have partial concealment (-2 to attack rolls) against adjacent creatures and total concealment (-5 to attack rolls, no opportunity attacks, can make stealth checks to hide) against non-adjacent creatures.
Note that concealment only applies to melee and ranged attacks. Close and area attacks are not affected.
