CSS stands for Cascading Style Sheets. CSS is the markup language that makes plain HTML look a specific way.
GitHub is a Git repository hosting service. Repositories are organizations of files. In this class, we use Github primarily for online hosting (put files online) and version control (go back in time if we need to).
See also: Getting Git Right
HTML stands for Hypertext Markup Language. Its ability to contain hyperlinks differentiates it from normal text.