Rooms in the Basic Game
The default type of room has no special symbol on the map, and this type of room makes up the majority of the map. They can have up to four adjacent rooms, which can have any type. Most of them will contain monsters 'indigenous' to the current chapter. Upon clearing such a room, a player might be rewarded with a random pickup or chest that will spawn in the center of the room (unless there's an obstacle like a stone, in which case it will spawn at the nearest free space to the top left).
Aside from the generic monster rooms, there are also some notable types that are still somewhat different: They are mostly void of monsters, and for example present an object like a chest, or a game of chance in the center. As with every common room, it is possible for them to exist multiple times on the same level, since technically speaking they are no special kind of room.
Rooms that will always appear:
Rooms that are always present in the first 3 chapters
Rooms of that type added in Wrath of the Lamb
|Super Secret Room|
|Challenge Room (may appear in Sheol)|
|(may appear in Sheol) Mini-Boss Room|
|Rooms that may occasionally appear|
Rooms of that category added in Wrath of the Lamb
|Rooms that need special conditions to be met to appear|
|Rooms of that type added in Wrath of the Lamb|
|Rooms of that type added in the Eternal edition|
|Eternal Challenge Room|
|Rooms in the Wrath of the Lamb|