Starting with HTML 4.0, in an attempt to return HTML back to a pure content mark up and separate out styles, cascading style sheets were introduced and CSS declaration syntax in HTML was standardized.
CSS declarations are pretty straightforward — one or more key-value pairs, separated by semi colon in curly braces applied to an HTML element. The HTML element is called the selector. More here http://www.w3schools.com/css/css_syntax.asp
Now, the tricky part on selectors. The hash and the dot.
You can restrict CSS formatting to a HTML element with a specific ID by using the #ID syntax.
E.g., if you have an HTML element called para1, you can restrict CSS to #para1.
On the other hand, if you want to apply the style to all elements, such as Center element in HTML, use the .Center.
Again more info here http://www.w3schools.com/css/css_id_class.asp