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The idea of Popcorn is to allow you to get your media onto your playback device, whether that be an iPod, iPhone or even a DVD. It is made by Roxio for the Mac and is ultimately very good, though I do not see the point of paying £30 for this, when for an extra £20 or so, you can get the more fully featured Toast Titanium. I suppose if all you want to do is work with video files, then it could be for you.
So, the first use if DVD copying. If you have non-encrypted DVD content, this could be a disk or files on your computer. Popcorn allows you to easily organise, compress and record onto DVD. You can also process these same files for the likes of an iPod touch or iPhone at the correct resolution and file format.
If you have lots of recording from a device like TiVo or perhaps Elgato, then again Popcorn allows you to get this into the format of your choice. There are loads supported including iPhone, iPod, AppleTV, Sony PSP, XBox 360, PS3 etc. the user interface is really nice to use, which sets it apart from other applications (some of which are free). There is also a nice disk label application, so you can make your creations look all pretty.
So, this is a GOOD application, not GREAT. If you want something that does a little more, then spend the extra of Toast Titanium.