Jekyll is a "batteries-included" framework that allows you to create simple websites quickly and easily. It describes itself as "blog-aware", focuses on simplicity, and produces static websites ready for deployment. It's a great choice for Github Pages integration, as Github Pages are written in Jekyll themselves.
- Simple, doesn't make too many assumptions, straightforward templating engine
- Great documentation
- Most popular static site generator at the moment, so plenty of community activity
- Lots of plugins for extensibility
- Batteries included
- Slower to build than some other SSGs
- Setup is slightly more convoluted than Hugo
It's similar to Hugo in many ways, but Jekyll's focus is more on simplicity compared to Hugo's focus on quick builds and being more content-driven. Templating is probably easier in Jekyll overall, but Hugo gives more out the box; it depends what you're looking for in your finished static site.