HTML is the markup for a website, the content. CSS is used to style the website and make it look pretty.
Semantic tags are used for accessibility, for those who rely on external devices to read content from a web page.
HTML can provide some formating with tags, it uses CSS for these effects, however the developer can override them.
HTTP (hypertext transfer protocol) is used to sent HTML over the internet.
Attend all classes, review lectures, do assignments, and practice on a daily basis.