Here is a brief explanation of what you will find in different classes:
AC First Class (1A): Offers air conditioned accommodation for two berths (one up & one down) or four berths (two up & two down) lockable compartments for overnight journeys; linen/ blankets; facility of shower/ toilet at either end of the carriage & attendant services are provided.
AC 2 Tier (2A): Air conditioned corridor carriage that sleeps 46 people split up into two & four berth cubicles separated by curtains.
AC 3 Tier (3A): Air conditioned 3-tier sleeper. Carriages are divided into compartments with six beds in each. The beds are stacked vertically in three tiers on either side of the compartments. During the day, the middle beds must be folded down flat against the compartment walls to allow passengers to sit on the lower beds. Two tiers of beds are also located outside the compartments, along the aisle. Toilet facility at either end of the carriage.
First Class (FC): Offers non air conditioned accommodation for two berths (one up & one down) or four berths (two up & two down) lockable compartments for overnight journeys; facility of shower/ toilet at either end of the carriage.
AC Chair Car (CC): Air conditioned carriage with reclining seats. Only sitting accommodation is provided.
Sleeper (SL): Non air conditioned Sleeper class. Similar to AC 3 tier except that it is non air conditioned.
Second Sitting (2S): Second class sitting. It provides only sitting accommodation with bench style seats.