One of the main benefits that Ello has over other social networking sites is that it's ad-free. That lack of advertising money is probably why the developers chose an open-source framework to design the site, and they use Ruby on Rails as their programming language.
In essence, it's very similar to the main Ruby setup that a lot of people are familiar with, but it extends on those foundations, adding in elements of CSS, Javascript, and HTML, with the intention of helping designers and developers to build their websites.
If you're interested in doing some of your own programming using the same system, you should find this tutorial really helpful in getting started. Because the video is a couple of years old, the version of Ruby on Rails they're using is slightly outdated, but the basics on display here should be more than enough to get you going!
