HTML is stand on Hypertext Markup Language and CSS is stand on Cascading Style Sheets, these two are core technologies for build website pages.
HTML is the language for describing the structure of Website pages and it is uses to publish online documents with headings, text, tables, lists, photos, etc. It is not a programming language, it is markup language. A web browser is read HTML documents and presents them as website pages because browser does not show the HTML tags, but uses the tags to interpret the content of the page.
Check this link to get info about http://en.wikipedia.org/wiki/HTML
CSS is the language for describing the appearance of Website pages, together with colors, layout, and fonts. It is used for describing the look and formatting of a document written in a markup language. It allocates one to adapt the presentation to dissimilar kinds of devices, such as big and small screens, or printers.
Check this link to get info about http://en.wikipedia.org/wiki/Cascading_Style_Sheets