According to Bing (Lol that rhymes!)
"3GP is a multimedia container format defined by the Third Generation Partnership Project for 3G UMTS multimedia services. It is used on 3G mobile phones but can also be played on some 2G and 4G phones."
Open With: Apple Quicktime Player / VLC /
"Compressed audio format developed by Ericsson; used by many 3G cell phones for voice recordings such as MMS messages; incorporates the Algebraic Code Excited Linear Prediction (ACELP) algorithm, which is designed to efficiently compress human speech audio." Open With: Apple Quicktime Player / VLC /
Both files are in the Binary format.
I don't see a lot of differences here.
Srcs:
http://fileinfo.com/extension/3gp
http://fileinfo.com/extension/amr