The 3gp Format
The 3gp format is a multimedia container format defined by the the 3GPP project for distribution and playback on mobile devices. The extension used for this file format is either .3gp or .3g2. Video streams are encoded using parts of the MPEG-4 standard and audio streams are encoded using either one of the AMR standards (AMR-NB, AMR-WB, AMR-WB+) or for higher fidelity, AAC. An excellent tool for converting .avi or .mpg videos to this format is the open-source tool ffmpeg.