Interlaced video is where an image frame has been acquired in two passes, known as fields.  One field contains the odd lines of the scene, and the second field contains the intermediate (or even) lines. Progressive video formats scan every line on every image.Deinterlacing is the conversion of content from an interlaced to progressive video format.

The deinterlacer is usually the first step in any format and frame rate conversion processing, so it is absolutely critical to retain the highest possible quality at this stage.  Any defects introduced at this stage will be compounded by onward processing, thereby damaging your precious asset.

It is essential to remember that the two fields that you are using to make one progressive frame are separated in time, so simply combining them can lead to very unpleasant artefacts.  Inferior deinterlacing methods can lead to jagged edges, ghosting and picture blur. 

A good deinterlacer will apply different processing in moving and stationary areas in order to obtain the maximum possible output resolution.