A geostationary transfer orbit is used to move a satellite from low Earth orbit (LEO) into a geostationary orbit. It is based on the Hohmann transfer maneuver.
The adjacent diagram shows a Hohmann transfer orbit to bring a spacecraft from a lower circular orbit into a higher one. It is one half of an elliptic orbit that touches both the lower circular orbit that one wishes to leave (labeled 1 on diagram) and the higher circular orbit that one wishes to reach (3 on diagram). The transfer (2 on diagram) is initiated by firing the spacecraft's engine in order to accelerate it so that it will follow the elliptical orbit; this adds energy to the spacecraft's orbit. When the spacecraft has reached its destination orbit, its orbital speed (and hence its orbital energy) must be increased again in order to change the elliptic orbit to the larger circular one.
This is interesting for us because the recent PSLV C-17 launched by ISRO positioned the GSAT- 12 satellite in the Geo-synchronous transfer orbit (orbit 2 on the diagram). From here commands will be given to a motor present in GSAT-12 to launch it further up into a circular geo-synchronous orbit (orbit 3 on the above diagram) at an altitude of 37000kms. Remember that geo-synchronous orbits can only be achieved at an altitude close to 35786 kms and directly above the equator.
1. http://en.wikipedia.org/wiki/Hohmann_transfer_orbit , http://en.wikipedia.org/wiki/Geostationary_orbit
2. he Hindu 16/711