The actual definition of purgatory is: A place or state of suffering inhabited by the souls of sinners who are expiating their sins before going.
Dr. Hatch throws whatever Glows have defied him down in Purgatory, the lowest level of the Elgen's compound as a form of torture. These Glows had refused to do ask Hatch asked and therefore he punished them. Ian was said to be down there for three years, before Taylor Ridley and Michael Vey showed up to help them escape.
The actual cell itself is said to be very dark, as the Elgen don't usually turn on the lights for the inhabitants. From what readers can gather, it seems to be a decent sized room with a lot of space and hard cold floor. Camera's are mounted inside to watch the prisoners as they sit in the dark and wait. The camera's are used to their advantage in Michael Vey: The Prisoner of Cell 25 as they start a 'fake' fight to get the guards attention to open the door.