obj is an old format from the alias wavefront days. It's actually the wavefront object format. Over time it ended up becoming standard to use it to transform object data between applications because there werent any good open sourced file formats for transfering data between programs. Every program had its own format, including obj btw. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.