Cascading Style Sheets. CSS.
CSS (short for Cascading Style Sheets) is the language of choice for styling elements of the internet.
The internet used to be a very cluttered place. One of the main culprits for all this clutter was HTML and the way it styled text and other content elements.
HTML as its own language is best suited to structuring text into paragraphs, quotes, lists and many other simliar uses such as inserting images and other multimedia into web pages. Until quite recently HTML also had the job of styling the individual elements. By styling we mean the colour of text or the size of any given header etc etc.
HTML very soon got messy
Forcing HTML to perform all these duties ended up rendering the HTML almost unreadable to the human eye as for every two or three sentances of actual content the HTML usually had the same amount, if not more, of cluttered style rules.
This may seem uninportant as when you view a webpage the HTML is hidden away anyway. To the human eye you can only see the text so what is the big problem?
Well, the search engines seem to want to read your content like humans too - and they can see all the HTML.
Dont confuse the spiders. They are your judge and jury.
The end result is that a search engine bot or spider (A little program that scans your site) will get tired of wading through all the HTML style rules and other clutter and simply abandon reading the page. Then there is a much higher chance that the page wont rank as well as it should - if even rank at all.
So, CSS to the rescue. With CSS, HTML is free to do the job it is supposed to do and the search engines bots can read your content freely.
Of course the above description is vastly oversimplified but it gets the point accross.
Website validation is your friend.
All this CSS and HTML can get very complicated to people who dont spend days working with it so to make sure you, the layman, know you are getting good clean code standards have been set. These standards are put into place via validation tests. You simply type the name of a site into the W3C validator and press go. A full instant report on your CSS or HTML.
Even though there are validators online co you can check your site once it is built - it is always best to know for sure that validation is guaranteed before you sign any contracts.