|In simple terms, it converts a video-type signal from one format to another. This could be conversion between two different TV formats (eg. PAL and NTSC), in which case the scan convertor is often called a 'standards convertor'. Or it could convert computer video signals from one resolution to another - and when the resolution you're converting to is a TV format, you've got a 'PC to video convertor'.|
Inside the unit, there's the ability to store the whole incoming picture in memory, and the ability to read it back out again at a different 'scan-rate'. The storing and reading-back can happen at the same time, so the whole system is 'real-time'.
Where the incoming and outgoing video resolutions don't match, pixels are added or removed as necessary - often without the user even noticing.