Prague has an excellent public transportation system (MHD in Czech). There are trams (streetcars), busses and the metro (subway). They all use the same tickets which can be purchased in the metro stations, in some shops, some hotels, etc. They cannot be purchased in the tram, bus or subway car. Follow English instructions on the ticket. There are some "night trams" which run only at night. Please make sure you validate your ticket as you enter the metro station or tram. These yellow machines are located on posts as you enter. An invalidated ticket is the same as no ticket and if checked by the metro personnel, there is a large penalty payable on the spot. One enters the metro area, tram or bus without having his/her ticket checked, however, the MHD personnell do routine checks often!
For more detailed information, please visit the official site of the Prague Integrated Public Transportation System
Below is a simple map of the metro system, however, all forms of maps are available on the official website above.