Cascading Style Sheets
CSS allows for complete control over the style of your html documents. It is a flexible and logical language, which is also easy to use. CSS is great for creating liquid layouts and for reducing file sizes by getting rid of html clutter.

Another great advantage is that with CSS your content is separate from your layout, making it easy to change the layout of your entire site all at once.

Class and Id
Class, which is preceded by a dot in the style sheet, marks elements as part of a group and can be used multiple times. An id, preceded by a hash mark (#) refers to a specific element that is used once on a page.

For both classes and ids it is best to use names that describe their function so you remember what the style refers to. If you name the class/id according to its appearance and later change it, the name won’t make sense anymore. It is also important to use unique names for classes and ids and avoid names that already pertain to styles such as body and table.

Links
When applying styles such as color and text-decoration to the four link pseudo-classes, remember to do so in the correct order or they won’t work properly. Link, Visited, Hover, Active. Think of it as LoVe Hate linking so you won’t forget. (Courtesy of CSS Crib Sheet).

Div and Span
Div and span have no inherent properties. This makes them great for applying classes to block elements (div) or inline elements (span).