What is HTML?
HTML uses special bits of programming language called "tags" to let the browser know how a webpage should look. The tags usually come in pairs: an opening tag defines the start of a block of content and an ending tag defines the end of that block of content. There are many different kinds of tags, and each one has a different purpose. See Basic HTML Tags below for tag examples. Some tags only work in certain browsers. For example, the marquee tag, which is used to make a bit of writing slide across the page, only works in the Internet Explorer and Mozilla Firefox browsers. Other browsers simply ignore this tag and display the writing normally. Many web page creators avoid using these "non-standard" tags because they want their pages to look the same with all browsers. This content was taken from Wikipedia
What is CSS?
Cascading Style Sheets, or CSS, are a way to change the look of HTML and XHTML web pages. CSS was designed by the W3C, and is supported well by most modern web browsers. The current version of CSS is CSS3. CSS4 is available, but is split into parts. One advantage to using CSS is a web page can still be displayed, even if the CSS is not working or removed. CSS code is saved in files with the .css file extension.