HTML and CSS are the modern building blocks of a basic Web page. Here is the basics.
HTML or HyperText Markup Language is the basis of everything on the web. You probably have some awareness of HTML even if you have no web design experience. It primary purpose is to organize and define the content of your website. Every web page you visit, is essentially an HTML file. (There are actually plenty of other files in most Web pages, they generally are either embedded in HTML files or masqerading as HTML files in order to communicate with your web browser.)
CSS or Cascading StyleSheets, is a newer edition to web design. It makes designing and styling your website much cleaner and easier to change then previous methods. CSS can either be in its own CSS file or it can be embedded in your HTML file. We'll discuss the pros and cons of both.
HTML = CONTENT
CSS = DESIGN
While I would be the first to argue that design is content, this is an important distinction to keep in mind as you design your website.