Files with the flv and mp4 extension are video files.
Files with this extension are compatible with multimedia players that can show video content as well as play audio content.
YouTube uses these 2 types of format for its videos.
The conversion from YouTube to mp3, in one of its stages, uses files in these formats, downloads the video (mp4 or flv), extrapolates the audio and creates the mp3 file.
Youtube2mp3 does not download the video files directly because these can be very heavy and take a long time to download; the conversion to mp3 (audio only file) permits fast download of the song.
The flv format, in particular, is created and developed by Adobe Flash Player.
Mp4 stands for mpeg-4 (Moving Picture Experts Group) and is the most recent version of the formats created by the Moving Picture Experts Group.
In brief, both flv and mp4 formats permit the reproduction of video content and the differences between them are purely technical: each format has its own distinctive technical features.
For example, video players can make use of these features to provide subtitles or to change the language.