HTML stands for Hypertext Mark-up Language, the language of the World Wide Web. It is used to create web pages and build websites. Consider using a cheat sheet such as this HTML5 Cheat Sheet to help you take advantage of all the tags available and build your projects more quickly.
CSS stands for Cascading Style Sheet. CSS allows web developers to specify the presentation of elements on a webpage separately from the structure of the documents. Here’s a helpful mini-course on CSS: Introduction To Cascading Style Sheets (CSS) thanks to HostingReviewBox.