HTML is the language that web browsers and web servers use to display web pages. HTML "tags," when added to a plain text document, instruct web browsers how to display that content on a web page. If a web page is a house, its HTML code is its blueprint.
If you want to give your readers the ability to interact with your site - to send information to it rather than simply receive information from it - you'll need to move beyond basic HTML. Forms are used to gather data from your users. When you use a search engine or enter a login name and password, you've seen a form in action.
CSS: Cascading Style Sheets
Cascading Style Sheets — even the name sounds a little intimidating, like perhaps you’re falling down a steep slope. But the name is just an accurate description of what CSS is — "sheets" (pages or chunks) of code that describe the style of a document in such a way that multiple commands "cascade" — in other words, they’re applied in a certain order that ensures that both the user and the page designer get as close to the same effect as they possibly can.