Every berth on a train belongs to a quota. General (GN) is the large default; the rest are reserved for a purpose: Tatkal (TQ) and Premium Tatkal (PT) for last-minute booking, Ladies (LD), Lower Berth and Senior Citizen (SS), Divyangjan (PH), Defence (DF), Foreign Tourist (FT), Duty Pass (DP), Head Office or Emergency (HO), and the Pooled (PQ) and Remote Location (RL) quotas for intermediate legs. Unused special quotas are released to general passengers at charting.
The quota you book in sets the waitlist you get. A GNWL ticket is drawing on the biggest pool; a PQWL ticket on one of the smallest. Choosing the right quota, where you are eligible, is the single biggest thing you control at booking.
Also written or searched as: quota list · GN quota · LD quota · SS quota · PH quota · DF quota · ladies quota · lower berth quota · senior citizen quota