HTML (Hyper Text Markup Language) is a widely accepted web-design language.
HTML is responsible for the construction, and the total output, of a page.
There are three "sections" of a decent HTML document source: Inline, Internal, and External.
Inline markup goes in the <body> tag, and defines the webpage's base layout. It structures the output of a webpage, such as creating tables and divisions (And the data in them), forms and buttons, links (And anchors), or even just normal text and images (And videos, flash documents, etc). Inline markup is not generally used to decorate the page with colors and borders, as these methods are usually deprecated. HTML should be used to create the structure of a webpage - Not decorate it with borders, lines, and colors (Although it's possible).
Internal markup goes in the <head> tag (Excluding DtD's, which go in the first line of a document, etc). Most of the time, users do not directly see anything in internal markup (With the exception of something such as the <title> of a page). Internal markup can declare some special things about a page that the user won't notice, such as the Character Set Declaration, the Document Type Declaration, and keywords (For search engine purposes) to that webpage.
Multi-web-language documents need to use Internal markup to link the multiple languages together (Like CSS and JavaScript), so in a way, Internal markup is also used for decorating, aligning (etc), and making dynamic things, out of Inline markup.
External markup is markup (Of any language) not contained in the HTML document, but in a different file. The contents of this file, to retrieve the markup, can usually be called by Internal Markup. External markup could effect the webpage in vast variety of ways.
CSS (Cascading Style Sheets) is another widely-accepted web-design language.
Unlike HTML, CSS does not "create" anything. Instead, it decorates, aligns, and positions (etc) elements in HTML. In a nutshell, CSS takes the normal HTML output and adds a few rules to how it's actually displayed.
CSS can edit things such as element width and height, background color, border, alignment, and actual visibility, for starters. HTML is capable of doing some of these things, but as mentioned earlier, the methods are usually deprecated, or are soon to be deprecated.
CSS is incorporated into a webpage using Internal markup (In the <head>; in <style> tags) or external markup (From a ".css" file).
