The Prague Public Transportation system is efficient and cheap. You have your choice of Tram, Metro, or Bus.
For information on getting to and from the airport and for the most up-to-date information on transportation in Prague, visit the official Czech Tourism website.
The Prague Tram System is a great way to get around town. Not only is it quick, but you get to see the city's sights as you cruise to your destination. Be especially aware of pick-pockets as they work especially hard at the crowded peak times.
The Prague Metro is a simple and efficient way to get around the city. It is divided in to three lines the A-line (Green-line), B-line (Yellow-line), and C-line (Red-line) that all criss-cross under the middle of town. For most tourist destinations you will probably prefer to go on foot, but if you're short on time and need to get across town to your next pub, go underground. Tip: Don't try for a free ride (especially on the A line). Ticket inspectors are looking for tourist and love to try and extract a fine (bribe?).
The Prague Bus System covers an extensive area outside of the town center. Most won't need to use it except for the trip to and from the airport.