The extension 3GP marks video files that are mostly generated by or intended for mobile phones. This standard was developed by 3GPP for 3G UMTS phones.
Like avi or Quicktime, 3GP is a container format. It is based on the MP4 container, but was adapted to the special conditions of mobile phones. So mainly the resolutions 128 × 96, 176 × 144, 220 × 176, 320 × 240 pixels are supported.
The video and audio data may be encoded in various compression methods. Subtitles are also supported.
The videos can also be protected with a DRM-like system. Then you need the respective, possibly paid license to view the content.
3GP files can only contain audio. For example, some phones use this format for ringtones. The Playstation 3 also generates this format for audio.
The support of the format by hardware players, be it phones or other devices, varies in severity. Most 3G-capable phones, but also some 2G and 4G phones support the format. How much depends on the capabilities of the hardware (storage space, display and camera resolution, etc.).
Many phones bring their own software for processing.