What's the difference between travis-ci.org and travis-ci.com?
Travis CI originally created two separated platforms to differentiate between private repos / paid (travis-ci.com) and public repos / free (travis-ci.org).
However, as of May 2018, new users and projects (both private and public) should only use travis-ci.com (see this blog post). Note that travis-ci.org will be closed down completely on December 31st, 2020 (see this newsletter).
Although still in beta—which is a bit weird since travis-ci.org will be shut down soon—Travis CI provides a well-documented migration guide.