There are two categories of train in Indian Railways:
Reserved Train: These are the major trains in India which run pan India and include every kind of trains (Rajdhani, Shatabdi, Superfast, Mail Express, Mail, Duronto, Garib Rath, Holiday Special, Janshatabdi, Luxury and Suvidha)
Unreserved Train: They are majorly Passenger trains and quite slow - moving trains, most of them have a stoppage at every next station of its route. The fare of these trains are very low and cost effective. The trains normally have unreserved seats but some overnight trains have sleeper and 3-tier AC compartments. These trains travel at the speed of 40–80 km/h (25–50 mph). So basically, one can sit and travel without reservation for the passenger train.