Movies are filmed at 24 frames per second. TV is 60 Hz, in other words it shows a new image every 60th of a second. In order for 24fps content to be shown on 60 Hz displays, a process known as 3:2 pulldown is performed.
In this process, the first film frame is shown twice, the second film frame three times, the third film frame twice and so on. In a film where the frames are labeled ABCD, the corresponding TV "version" after 3:2 pulldown is: AABBBCCDDD.
Why is it 2323 when it's called 3:2? After it was initially developed researchers found it was less noticeable as 2:3 instead of 3:2.
The trick is progressive scan televisions, which is all current HDTV models. In the process of making a 1080i image into a 1080p image can cause artifacts when the original source is based on film. Think of watching a movie on Sunday night, or even most scripted TV dramas and comedy shows, these are often shot on film.
Modern TVs and Blu-ray players do a pretty good job deinterlacing the image, pickup up the 3:2 sequence and correctly deinterlacing it.