FLV (Flash Video File) is a format created for transferring online videos. The FLV format is a Flash-compatible video file created using Adobe Flash or similar software. The FLV / F4V file format specification for storing media content is open source, which is also supported by many third party programs. Previously, files of this format were used to make it difficult to illegally download video content on popular video hosting sites. You can open a .flv file using third-party players that have all the necessary codecs in their assembly.