Telecine is necessary because of the differences in frame rate (the speed at which each individual picture is revealed on the screen) between movie and TV projection. While movie projection runs at 24 frames per second, TV runs at 30. Without Telecine, a movie shown on TV would have a flickering effect.
Telecine was originally used to present live recording for TV broadcasts. When the popularity of live TV programs in the early stages of TV increased (due to the previous popularity of live radio shows), Telecine was used to keep up with the live pace.
Telecine first works by slowing down the film by 1/1000. In this way, a movie that was originally shown over two hours would take an additional 7.2 seconds to show. This creates four original frames for every five new frames (called a section). By stretching each frame, the final frame in each section doubles over itself. Mathematically, the next section would result in the final frame having three copies. Thus, the resulting copy is the most common format in the United States, called 2:3.
The only use of Telecine to produce a DVD today is if the DVD was created directly from a video tape. Most DVD production maintains the original frame rate of the movie through an entirely different computer editing process.
Although very rare, Telecine can also refer to a type of piracy of movies done through the use of the same equipment used to legally create movies. To do so, the criminal must have access to the expensive equipment used in the process and the original movie print itself.