Is it an online learning environment? Yes!
An e-learning application? Yes (and much, much more)!,
More specified as a Learning Management System? Yes!
I did my Masters on Moodle back in 2011. The aim of my thesis was to see if the use of Moodle in school could increase self-directed learning among Diploma-level students. I would have touched on some aspects of usbility. You can read my thesis if it will helps. The link to my thesis is somewhere down the page of: https://moodle.org/mod/forum/discuss.php?d=169255