An EasyCap is a USB device that connects a composite video source (like an "analogue" cctv camera) to a computer.
The name "EasyCap" is not really a product name, because there are so many companies making copies of the original device. In fact one company (ClimaxDigital) got so fed up with the number of bad copies in circulation, that it change its branding to EzCAP, and more recently, dropped the name completely.
So you don't really know what you are getting when you order one of these. Its only when you connect it to your Linux computer and type "lsusb" that you discover what is inside.