FLV (Flash Video) is a video container format developed by Adobe Systems and comes with a .flv filename format. FLV is primarily used to stream audio, video, and text data such as subtitles on sites like YouTube, Metacafe, and more. It supports MPEG-4 Part 10 (H.264) video codec and AAC sound.
RealPlayer, Apple QuicTime Player MPLayer VLC Media Player
AC3 is an audio format designed by Dolby Digital to offer surround sound listening. This format is available on most Blu-ray and DVD media files and is played using a Dolby Digital DVD Player. Interestingly, AC3 can be output in up to six speakers of a home stereo system. On a computer, AC3 can be opened on VLC Media Player, Apple QuickTime Player, and Windows Media Player.
VLC Media Player Apple QuickTime Player Windows Media Player