FLV stands for FLash Video, and is a proprietary streaming video format from Adobe. This format is used to store and share the video clips on sites like YouTube, Google Video, Yahoo! Video, and many other sites.
Normally you cannot download the actual FLVs, but there are applications and hacks that let you do so. There are also several FLV players and converters (that convert FLVs to other movie/video formats) available.
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