HTML is a language used for content structure. CSS is used for the formating of the built in HTML content.
It sounds somehow complicated and technical. Let's make it a little easier to understand.
Several years ago, HTML language used to give structure to the text contained in the websites. A text writer for a website, could construct the contents using HTML "tags" as the < h1 >, which indicates a header and < p >, which creates the paragraphs.
As the Internet became more popular, web designers began to look for ways to change the appearance (layout) of web pages. To meet this demand, the manufacturers of browsers (at that time Netscape and Microsoft) created new HTML tags, such as < font > , which differed considerably from the original labels, as defined by the appearance and not the content's structure.
The same fact also led to a situation where many of the original HTML tags used wrong to format, and not for the configuration of the websites such as the < table > tag. Also, many of the new labels used for formatting supported only one type of browser, as happened with < blink > tag.
CSS invented to resolve this situation, providing designers of websites a large range of formatting options, supported by all browsers. At the same time, the split of the presentation from the content, greatly facilitated the maintenance of the pages.
