FLV stands for Flash Video its a video encoded to use by a flash player (even though now most players supports the playback of flv files).
Its usually very compressed and with a not so good quality. The most common codec used is H264 which is one great codec to compress your home videos.
To open you will need any of the most common and popular video players as GOM Player or Media Player Classic
Gom Player is one of the finest video player out there, its free, nice and fast. Want more reasons to download it? Try it! if you dont like it consider its free! :)
Go to this link to download the Software: http://www.gomlab.com/eng/GMP_download.html
Media Player Classic
Is one of the best video player, it main strength is its size and speed. It support most of the new codecs and splitters as MKV.
And if so far you are not decided... ITS FREE! :)
Go to this link to download the Software: http://sourceforge.net/projects/guliverkli/