.3gp and .3g2 are file extensions for video in 3GP format. This is a multimedia container format used by mobile phones. 3GP is a simplified version of the MP4 format and was designed to make file sizes smaller so mobile phones could support video.
There are two standards for this format: 3GPP, with the file extension .3gp, and 3GPP2, with the file extension .3g2. 3GPP and 3GPP2 stand for 3rd Generation Partnership Project and 3rd Generation Partnership Project 2 respectively. These were separate collaboration agreements between several countries standardizing mobile phone formats. 3GPP files are for GSM based phones, 3GPP2 files are for CDMA based phones.